What and how would you do it?! This is a HUGE general question.
For me:
If I could do it over again, I would have never studied architecture and would have went to med school to become a Research Lab Technician.
If I could do it over again, I would have never sold my t-red CSVT, it was paid for. 
If I could do it over again, I would have accepted a manager position for a good buddy of mine's band, now called "The Killers".
If I could do it over again, I would have never came back from "Chicken Wing Zing 2005" and just lived there.So, lets' here what you guys wished you would have done, it's never too late to make changes in your life, but sometimes....you're just a few seconds shy of getting aboard the boat.