Early life and death of Goku
Future Yamcha's life was exactly the same as his main timeline counterpart's life until the Trunks Saga. In August of Age 764, Yamcha watched as Future Goku used the Instant Transmission to arrive on Earth and kill Future Frieza and Future King Cold (something Future Trunks' arrival in the main timeline alters). Sometime after this, Future Yamcha and Future Bulma breakup, and Bulma has a baby with Future Vegeta, Future Trunks. Two years later, in Age 766, Goku died from a lethal Heart Virus, and is unable to be wished back due to the death being of natural cause. Yamcha is shown outside Goku's house when Goku tragically died of the Heart Virus.
Battle Against the Androids
On the 12th of May, Age 767, two killer androids would appear six months after Goku's death of a heart virus. Coming to the defense of Earth's inhabitants, Future Yamcha and the other Z Fighters would engage the androids in battle. Future Yamcha was the third Z Fighter killed by the androids; he dies after Vegeta and before Tien and because of Piccolo's death, the Dragon Balls vanish permanently as the Z Fighters are unable to be wished back, which also kills Kami.
In the TV special prologue, Yamcha is shown receiving a powerful kick to the neck by Future Android 17, and in Future Trunks' flashback in "Ghosts from Tomorrow", it is shown that 17 killed him by blasting a hole through his chest with his Photon Flash. The only survivor of the battle is Future Gohan, who dedicates his life to avenging his fallen friends.