I would refrain from asking the girl I went to prom with out on a date afterwards, since it turned out she was moving to California.
I would have asked a few girls out (but of course, I say that now...), or one if it worked out.
I probably would have not taken AP Calculus BC in high school, because despite doing my absolute best and passing the exam I didn't get a good enough score to get college credit for it, so it ended up being a colossal waste of time (I'm a music major).
I would prevent myself from getting hit in the eye with a golf club in elementary school.
I would change my name. It's dull, uninteresting and mispronounced so much (my first name, Brennan, is constantly pronounced Brendan, Brandon or even Brian, and it drives me insane.) I want to pull all my hair out. I would change it to something that sounded cool, with a first name impossible or at least very difficult to mispronounce, and an awesome last name that is cool and unique enough that it actually sounds good when you name a company after your last name ("Johns?" for one, that already exists, and everyone instantly knows that's just some guy's last name.)
Other than these insignificant things I would change, I love my life and wouldn't change anything else.