This is a really loaded question I feel like. Going back in time would mean losing the good things in my life now, presuming I did things differently, and considering that I wouldn't press the button at all as I am happy with where I have come and where I am hopefully going.
If I you were to have asked me this question as recently as a year ago I would have told you I'd have wanted to go back to when I was 11 or so. In fact, I would say it was something I dreamed for and wanted more than anything in the world to be able to do. Mistakes I thought I had made is what I blamed for all my problems, and were problems within themselves, so I would want to go back and stop myself from making them ever.
Now, knowing all I know and going back now, if given the chance I may consider and try to use to my advantage. I may fear it would backfire on me, but basically I would be all knowing of the future and control it in anyway I wanted to. The ethical complications of this would make me nervous, as basically I would be manipulating all the people around me and making what I wanted to happen to happen. I may overlook this if I considered myself a moral enough person who would not hurt others in my new formed path.
Having the intelligence of a 21 year old as a 12 year old would also make me wise beyond my years, and a appearing to be somewhat of a genius. I would be able to get into a prestigious school, probably on scholarship. I would be able to know how to work my body to be good at sports, and then excel at that as well. And socially speaking, I may be able to live the "childhood I never had" as I was too focused on the mistakes I made to do anything about it.
But that would all change who I am, and there's not anything wrong with me when it comes down to it. I know this is just a silly hypothetical question but I have moments where the past saddens me, the things I missed out on and the things I could have done differently. The fact that I can't redo things, or change things about the past. But the present is all that really matters, and I said before there is good in it. That means that the past, as imperfect as it was, must have done something right.