In the final confrontation Harry is crippled. He therefore invokes a spell in 2010 that sends him back into the dieing body of Henry Potter – James younger twin. The Potter and with them a great part of the order were attacked in May 1979. Harry’s powers and knowledge outlive the transition and he is therefore in a position to destroy the five Horocrux up to date and kill Voldemort in 1973. As a by product he gets involved with Ellen MacGregor and improves the wizarding world slightly.
The story can be read at FicWad.
There is also a PDF.