turning point (1)

With time travel, if you change one thing, everything after that will change too. That in mind...

If when you die you are reincarnated as yourself, the same date, the same time, the same everything, but were "instructed" to change one thing (as in something that you could change, not asking to be born as a cat or something along those lines, I mean change one thing as in change one thing you did in this life that will, hence, change the course of your next), what would it be?

(To be clear, make it a given that you live the same life over and over and over again, but have to change one thing each time around. For your next time around; what will be the one thing that you do differently to change the course of your life?)