The geek in me would want to go waaayy back to like Vikings or renaissance period to see what the old writings never told us like, r there dragons. ( I know I’m weird it is just what I would do).
I would go back to November 2013 and change the details in the Marvel/Netflix contract so that the Defenders' shows were run by Kevin Feige rather than Jeph Loeb to make them cannon to the main series
I kind of also want to go back to the day Princess Diana got into that "accident". I could go undercover and listen to prior conversations, watch the incident from different angles, finally get the truth and expose the Qu--perpetrators!