Hitler didn't do his thing in a vacuum. There was an opportunity for him, circumstances, values of the time, the trauma to Germany what happened in the first world war and after it, competing ideologies like communism, political instability in Germany, inflation, recession etc.

So maybe the person could go back in time and discuss with the negotiators at the Versailles treaty after the first world war. Or maybe try to inject some economics education to try to prevent the hyperinflation.

Room for some XKCD or SMBC comics!