dragon ball z - How is the future changed when Trunk goes to the past and killed Frieza and helped Goku?

Trunks goes back to the past and killed Frieza. He also helped Goku with heart disease.

Trunks described that there are now two parallel universes: one in which Goku is alive and another where all warriors are dead. And then the story continued in Goku space. What happened to Trunk's world from where he came from?

And when Trunk killed the androids and going back again in past then Cell kills the Trunks and takes over the spaceship and goes back in past to merge with Android 17 and 18. But how the fight between Trunk and Cell is not shown in detail? And how Trunks reaches at that point?

Answer

Timeline 1

Cell3 arrives from the future from the Time Machine he stole from Future Trunks3 in an egg form because he could not fit. He hatches and burrows underground to develop.

Frieza and King Cold invade Earth to get revenge on Goku, and to also destroy the planet. They are killed by Future Trunks2 who appears from the future.

Future Trunks2 then meets Goku when he arrives on the planet and gives him the Heart Medicine, from 20 years in the future; he also warns the Z Fighters about the Red Ribbon Androids who will arrive in three years, before returning to his timeline.

In the next three years, the Z Fighters go into intense training, and Trunks1 is born to Vegeta and Bulma as their first child.

Dr. Gero (aka Android 20) and Android 19 battle the Z Fighters. Goku, who did not take Trunks' medicine/antidote, becomes sick during his fight with Android 19. Vegeta arrives and takes over, destroying 19, and Goku is taken home and given his medicine. Dr. Gero flees.

Future Trunks2 arrives back to this timeline and finds the destroyed Android 19, a robot he has never seen before. He begins to realize that this timeline is starting to branch off considerably from his.

Dr. Gero escapes back to his lab and awakens Android 17 and Android 18 who then proceed to kill him. They go on to activate Android 16.

The Z Fighters are defeated by 17 and 18, but none are killed. Future Trunks3' Time Machine from timeline 3 (used by Cell3) is found, along with Cell's shed skin.

Cell3, now fully grown, starts razing cities and absorbing the people to raise his power. Piccolo fuses with Kami and battles against Cell3. Cell tells Piccolo about who he is, why he is here, and how he got to this timeline. Cell escapes after Future Trunks2 and Krillin arrive on the scene to aid Piccolo.

Piccolo hunts for Cell3 while Future Trunks2 and Krillin go to Gero's underground lab to destroy Present Cell1 while he is still gestating. While there, Future Trunks2 finds blueprints for the androids which he gives to Krillin to take to Bulma.

To become stronger, Vegeta and Future Trunks2 train in the Hyperbolic Time Chamber. Piccolo, unable to find Cell, is confronted by the androids. He decides to attempt to destroy them in order to prevent Cell from absorbing them and becoming stronger.

During Piccolo's battle against Android 17, Bulma contacts Krillin telling him that she is created an emergency shutdown device based on Gero's blueprints. It has a range of 10 meters so Krillin rushes to intercept her and obtain the control device to deactivate and destroy the androids.

Cell3 detects Piccolo's raised ki and deduces that he is battling an android; Cell rushes to the scene to absorb the androids and complete his development.

Cell3 interrupts Piccolo and 17's fight, quickly dispatching Piccolo and nearly absorbing 17 before Android 16 intervenes. Cell and 16 fight relatively evenly, but in the end Cell manages a surprise attack that allows him to absorb 17 and become Semi-Perfect Cell.ell proceeds to easily deal critical damage to Android 16 and is going to absorb 18 but is halted temporarily by Tien Shinhan, allowing 16 and 18 to escape. Goku quickly arrives on the scene and rescues Piccolo and Tien before retreating again.

Vegeta and Future Trunks2 emerge from the Hyperbolic Time Chamber, letting Goku and Gohan enter. Vegeta battles against Cell and is clearly dominant in the fight. While they are fighting Krillin approaches Android 18 with the shutdown device, but is captivated by the beautiful android and destroys the remote. Vegeta quickly becomes arrogant with his power and gives Cell leave to absorb 18 to attain his perfect form.

Cell finds 18 and finally absorbs her, becoming Perfect Cell; he then goes on to defeat Vegeta and Trunks. Cell decides to hold a tournament, the Cell Games, in ten days to test his new abilities.

Goku and Gohan emerge from the Hyperbolic Time Chamber. Trunks, Vegeta and Piccolo all use the room before the tournament starts. Goku recruits Dende to serve as the Earth's new guardian and reactivate the Dragon Balls.

The Cell Games start and Goku fights with Perfect Cell. Goku eventually forfeits the match and chooses his son Gohan fight instead.

Gohan fights Perfect Cell and, after eventually being provoked into transforming into the Super Saiyan 2 stage, delivers a fierce set of hits to Cell, causing him to regurgitate Android 18 and transform back into his Semi-Perfect state.

Semi-Perfect Cell attempts to destroy the Earth by self-destruction, but Goku sacrifices himself to teleport Semi-Perfect Cell from the Earth to King Kai's planet. Cell explodes, killing Goku and King Kai.

Cell's regenerative organ miraculously remains intact, allowing him to regrow himself. Cell returns to Earth, stronger than ever and kills Future Trunks2, but is defeated for good by Gohan.

Future Trunks2 is revived by a wish made to Shenron, and returns to his timeline. The rest of Dragon Ball Z happens.

Timeline 2

Frieza and King Cold invade Earth and are defeated by Goku. Trunks is born, Goku dies from Heart Virus.

Dr. Gero activates Android 17 and Android 18. They kill him and then proceed to kill all of the other Z Fighters except for Gohan. The Dragon Balls permanently disappear, after Piccolo's death.

Gohan becomes a Super Saiyan.

Over the course of 13 years Gohan challenges the androids alone but never wins. Gohan trains Trunks, but after loosing an arm is later killed by the androids. Trunks attempts to avenge Gohan but is badly beaten.

Bulma builds a Time Machine to send Trunks to the past with the hopes of finding a way to destroy the androids. Trunks takes medicine to cure Goku's1 heart condition with him. Trunks goes back in time to timeline1.

He returns after killing Frieza1, giving Goku the medicine, and warning the Z Fighters about the androids.

Trunks goes back in time again to help the Z Fighters in timeline1 to combat the androids, and later Cell3.

Trunks returns from timeline1 more powerful as a result of his battles and training. He quickly finds and kills Androids 17 and 18 with little effort.

Trunks kills the newly emerged Cell2, knowing about Cell's plan to kill him and go back in time (see timeline 3). Earth slowly rebuilds its cities as peace comes to its inhabitants.

Timeline 3

Frieza and King Cold invade Earth and are defeated by Goku.

Trunks is born, Goku dies from heart disease.

Dr. Gero activates Android 17 and Android 18. They kill him and then proceed to kill all of the other Z Fighters except for Gohan. Gohan becomes a Super Saiyan.

Over a course of thirteen years, Gohan challenges the androids alone, but he can not defeat them.

Gohan trains Trunks, but is eventually killed by the androids. Trunks attempts to avenge Gohan but is badly beaten.

Bulma builds a Time Machine to send Trunks to the past with the hopes of finding a way to destroy the androids. Trunks takes medicine to cure Goku's4 heart condition with him.

Trunks goes back in time to timeline 4 (which is created because of this trip). He returns after killing Frieza4, giving Goku4 the medicine, and warning the Z Fighters4 about the Red Ribbon Androids.

Trunks goes back in time again to help the Z Fighters4 in timeline 4 to combat the androids.

Trunks returns from the past and destroys the androids after using their emergency stop circuit.

Cell emerges and, realizing the androids are gone, decides to use Trunks' Time Machine to go back in time and find the androids in the past.

Cell ambushes and kills Trunks, gets the Time Machine and goes to Age 7631. This eventually creates timeline 1 and 2. In timeline 1, Cell is destroyed by Super Saiyan 2 Gohan1 with a Father-Son Kamehameha.

Even though Trunks died in this timeline, both Cell and the androids are gone, so Earth presumably starts to rebuild itself.

http://dragonball.wikia.com/wiki/Alternate_timeline