I'd go back to the time when I declared my major field of study in college. Instead of declaring business economics, I would have chosen to be a History major and Poli. Sci. minor. Both subjects interest me a LOT more than finance and accounting do. I would have sought to become a teacher of US & European History and Geography.
But...as Indianapilot correctly states, it's very difficult (if not impossible) to change careers once you're pretty far along one path. For me, I'm now trying to get into grad. school and pursue international business, which will hopefully allow me to spend time in Europe and Germany specifically.
I'd also want to go back earlier, to when I was younger and try to stand up to the people who picked on me a lot. Even if I didn't prevail, it would be better than just running the opposite direction and doing nothing. I still do the same thing today but in different situations...not good.
And the winner for best actress is....REESE WITHERSPOON for 'Walk the Line'!!!!!!!!