I don't mean historically as in kill a world leader etc. I mean one thing that would change yours or family/close friend's life.
Mine would be force my sister to get her pain looked at. She left it years popping pills and died of something that would probably have been prevented if she'd gone to the GP sooner. Although to be honest they were giving her strong pain relief. I'd have made her get exploratory tests. She died too young. I used to beg her to go. If I could go back I'd drag her there and be forceful at the appointment.
Is there anything you'd do?
I've made a lot of mistakes in my life I could choose to rectify if I could go back. But nothing that has caused harm.