In my own life, I'd go back to the late 90s and stop my coworker from committing suicide. I'd then be a friend to him.
I'd then go back to the late 70s and insist on taking self defense lessons, preferably Krav Maga. It would've made a world of a difference in my life.
I'd go back to 1968 and lock up my parents so they wouldn't get married at 18 & 19. I'd insist they take marriage and parenting classes first.
In my fathers young life, I'd insist he learn self defense also. And that he not develope the smoking habit that eventually killed him at the age of 63.
In my Aunt's young life, I'd insist on getting her treated for polio immediately instead of waiting as long as they did, resulting in her having a paralyzed leg all of her life, plagued with misery.
I'd go back to other relatives young lives and protect them from being molested.
Speaking of getting molested, I'd go back to when I think I might have been molested, and make sure it didn't happen.