There are two current theory for how time travel would be possible, the first is to travel at or near the speed of light. This causes time to pass more slowly for the person at light speed while everything else flows at the normal speed. The trip is one way though and only allows for the explorer to travel forward in time.
The second is one that I'm not as familiar with since it's a little more out there. Basically you take a black hole and wrap it around itself on an helical axis. This supposedly distorts space-time in such a way that a person can travel back in time, however they would only be able to go back as far as when the black hole was tampered with (assuming you would survive the spaghettification of the black hole).
Or you could always try to get your hands on one of these:
