Assume with me that the universal passage of time is merely a set of state transitions from A to B controlled by guiding principles and that everything is entirely determined so that even if you were able to back time up, if you were who you were then things would still come out the same.
Ok, now, understand that if you don't like my assumptions you can buzz off as the rest of the topic relies on them.
Now, the next bit of set up is that you need a teleporter or you know, one of those things on start trek that can make any food you're capable of programming into it. Only, you need a really big one capable of rearranging all of the molecules within range of it into whatever pattern it decides it needs within the range of several light years. The actual physics of this is not entirely relevant, but the gadget would create an event horizon wave that mutates anything which comes inside of it into what needs to be there and it would expand for all time from the epicenter at which it was created, thereby rendering itself as a new reality.
Now the idea is that you know exactly how a specific time period was set up down to the last interstellar particle X years ago and you want to exist X years ago. So you set off this machine. It copies over the current universe replacing it with an old version, just like a hard disk and places a copy of present day or future day you in the recorded copy of the old universe.
Wa la.
