i would go back in time and shout at the teachers who gave me annoying homeworks last year just to give me homework and as another punishment give them a quiz about detective conan with 10,000 questions and mark them all wrong even if a few are right
then i would go into the future and find out who it is i marry (that is if anyone marries me) and see what my future kids look like and find out their names and find out who they are in love with if i have any kids and they are in love with someone