Let us suppose you receive supernatural powers out of blue and now you have decided to go back to the past, you went to the past successfully and you try to change something for good. What would you change and why?
For me I will go to the past and change my habit of smoking coz smoking is killing my mouth and I am afraid of mouth cancer now.
Most of us would go back and mine bitcoin :p
Anyway I would go back and somehow pass my real me some messages how to beheave and how to respond. Because when I look back like 10years from now I done nothing important and missed a lot of good opportunities and today I am struggling a bit but I have learned my lesson.
Positively for me I don't have that smoking habit, it's good that you admit that. I'd suggest that you hang out with people that want to and with once that have successfully quit smoking because you want to feel them and motivate yourself to stop. You can find those people in real life or online on the various forums.