Okay, I have a wife and kids, and can't do anything that risks their existences, so how far back I go is limited.
The earliest I can safely go back is mid-2010, so that's when I start.
For my first wish, I'm going to give the DA in New York incontrovertible evidence of the size and value of various properties in and around NYC as reported to banks for the purpose of securing loans, and as reported to state tax authorities for the purpose of determining taxation.
That should be enough to get Donald Trump indicted for tax fraud at a time when we still had a functioning judiciary, and before the red wave election in November.
For my next wish, I'm going to Twitter and giving them the easy ability to identify and block foreign disinformation agents on the platform. This is well before Twitter decided it liked the clicks Russian trolls bring. End result: The most widely used platform by Russian trolls is cut off to them, and Twitter execs have evidence of foreign interference efforts to give the feds early.
Finally, I'm zipping forward to 2012 and giving the two guys who tried to assassinate Putin some better planning, opsec, and training. Couldn't hurt.
You're welcome, everyone!