Seems too dangerous to change things before you are born. You know, the whole space-time continuum thing.
Actually, to hell with it, I wouldn't change a thing. I suppose there are more than 2 girls that I could've/should've banged but didn't. I could say I'd go back to when I was 12 and slap the first 40 I ever drank out of my hand, but I probably would've just drank another one soon enough thereafter.
Wait I have a sure fire one, I would've not hurt my back wrestling when I was 17. I was wrestling a guy bigger than me who was made of muscles and was a former Tokyo prefecture judo champion (though this was in the US), and I got smashed. That's one I would like to have back.
And maybe I would've done my road work properly before my first and last amateur boxing match when I was about 20. That makes 2.