The publishing of Twilight. I don't want to screw with anything to big that could have huge negative effects. I would assume getting rid of Twilight would only coast the world a loss of T-shirts and other silly products.
I would have killed Ghengis Khan before the Mongols could have wrecked Asia. If they had not taken over China, we would right now be meer advanced than we are now. China was so many hundreds of years ahead of the Europeans and where not far from going out to explore the Americas before the constant threat of Mongola set them behind. We would all be speaking Chinese now if that had happened, but we would have been talking about how our granparents where so behind in technology with their dragon inspired floating cars.
Going back and time and stopping something in history will create a paradox. If I go back to stop something, it didn't happen, so I don't have to go back and stop it so then it happens and I do have to go back and stop it. Then the space-time continuum would be all messed up because of the paradox and potentially pose a lijst of theorized problems.
So I wouldn't change anything. Besides, bad things happen for a reason and always have some sort of bright side for the future no matter how dark and dense they are. We learn from bad things, and good things can come out of the lessons we learn. Why change any of that?