Basically, prevent the hormonal treatment from happening to him. Whether that means him running to the United States, or getting rid of the law banning homosexuality in the United Kingdom, find a way to prevent Alan Turing's suicide at 41.
Additionally, could he have contributed more if he was able to keep on living? He is a father of computer science and artificial intelligence. What could he have done with more life? Would computer science be much further advanced?
Now, begin! Save a genius, and one of the most important scientists of the 20th century.