To go back in time and stop the Hiroshima attack. When I was 9 I felt like shit after hearing that the US had done that. Killing thousands of civilians for nothing. Just because they happened to be in a city were they had grown accustomed to. It pisses me off to this day. I've always old myself I would end up moving to Hiroshima myself to get out of this hell hole we call America.
A Death Note. I would just use it for the lulz, though. If the President of the USA ever goes out in the street, has anal sex with a horse, and then gets hit by a truck, you'll know my wish has come true.