There are many ways you could change the past, some big and some small. You could kill hitler before the holocaust. You could prevent families from getting on the Titanic. You could tell medical profressionals the cure to diseases that weren't cured at the time. You could set two people up that wouldn't have ended up together otherwise. You could tell your parents a future winning lotto number. You could give someone the idea to invent something that will turn into a multi-million dollar company someday.
There are really endless possibilites, but those are the few that came to mind just now!