Haibara's Teenage New Game+

When socially anxious college senior Natsuki Haibara thinks back on his high school life, all he has are fleeting fantasies of a happy adolescence that could have been. Imagine his bewilderment and surprise, then, when he inexplicably finds himself seven years in the past—one month before his first year of high school. He'll need all the help he can get to succeed, from a workout regimen to online how-to guides, a childhood friend, and plenty of sheer willpower.

Synopsis from Anime News Network