It depends on whether I will just be able to visit and view that period like looking through a camera lens or actually go back in time and change few things or decisions that I had made in past which may have better impact if had been done differently.
If only visit the period as if looking through camera lens :
I will visit my high school years,because I believe they were among the best of my life. I studied a lot, played a lot and enjoyed thoroughly during those years. I have made best of my friends during those years, had my first crush, played hockey at State level. During those teenage years we tend to do everything with lots of passion and zeal and I would like to revisit that energy and incorporate at least a part of it into my current life.
If I could actually go back in time and change few of things:
I will visit my graduation days specially the second year of my college where I was given a impression that it doesn't matter how much grades you get or even that a degree is not necessary to succeed in life. So I had joined a group of YOLO mentality batch-mates, partied a lot, watched every every TV series ever produced and almost skipped an entire semester. As a result I ended up having four back-logs in a year which had a 12 course syllabus. And the courses which I cleared, I did with just passing grades in most of those subjects.
I was so ashamed of what I had done but when you are in hostel it becomes very difficult to change your friend circles or also the way you have accustomed yourself to spend your day.
But I did recover in my final years and eventually passed my degree with First class even though I still have doubts in some basic concepts of Mechanical Engineering which were taught in the year I almost wasted my education fees.
So if given opportunity I will visit the second year of my college and throw away the illusion created around me and start in studying without any doubt in my mind. In that way I wouldn't have missed out in attending other interactive courses on different interesting subjects or back out from participating in annual and sporting events because I had to study.
I missed and had to sacrifice many things in my final years including a date, because I had to study to clear my backlogs I had in my second year.
Also I know this is the nerdiest answer you can get for this question.
Sources for images used:
https://pixabay.com/photo-851328/
https://pixabay.com/photo-3304444/