It's pretty much impossible to go back and change anything. If you go back and kill Hitler before his rise to power, you then will have had no reason to go back in the first place to kill him. If you could go back, that means you'd always have been a part of that past. You go back in time to kill Hitler, no matter what you do, you didn't succeed because history tells us that nobody stopped him until he killed himself.
The other theory is that it would spark an alternate timeline and if you went forward, you'd run into the alternate timeline you and it'd be impossible for you to go back to your timeline unless you went back in time again and prevented yourself from doing the timeline changing event.
Duh!
But if I could, I'd go back to when man was first starting to think outside of themselves. Where they were just starting to wonder their place in the world