Change the course of World War 1 by giving the British and French army the necessary knowledge. To get their respect I'd need to do something first, but once you have that...

Tanks, motorised columns, fast moving infantry. And the whole german plan known in advance, as well as the best ways to stop it. End the war in 1914 and save millions of lives and prevent WW2. No nazis, no communists, no balkanisation, no sudden collapse of the European empires, America remains relatively isolationist.

The fun part then is seeing how differently things develop and if they really could be better or if the 20th century is doomed to go badly.