I would change for the better the economical situation of the Roman Empire in 300 a.d., so that they wouldn't be in a crisis that would lead to malfunction in every branch of government and in the army, and that would influence the quality and quantity of Roman troops and people's cohesion during the next centuries, and thus changing effectively the world, because it would survive the destruction of the Empire at the hands of barbarians.
Yup, the world, because Middle East changed because of influence of both assholes from Europe and the Far East, the Far East during the last centuries had ties with Britain, that changed them forever, and America would be different today from what it is now, because, y'know, European colonisation.
How could you describe European colonisation methods?