You can go back in your mind and change memories you didn't like. Because memories are just movies we keep playing in our heads. Neither past nor future exists anywhere, except in our minds. So we can't go back since just now exists. Always. And just now we can find peace or joy, never in the past or the future.
I don't know about going back, but I think I figured out how to go into the future. I went to sleep, and I woke up tomorrow. It's crazy! :)
Seriously though, as others mentioned, there is no such thing. Every time you "retrieve" a memory, you're actually recreating it in your working memory, and experiencing it there. Equally so, when you're thinking about the future, you're just envisioning it in your working memory as well. Even further, your conscious mind is ALWAYS in a delayed state (roughly 82ms), so it is never experiencing "right now" or the "present," but a continual residual state of the past. There is no present, past, or future - simply your experience and idea of it all. As such, the only thing of any importance is your current perceived experience. Master your experience.