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The closer to the speed of light that you travel, the slower time appears to be for you in relation to everyone else. So if I were to zip around space for a few months at 95% of the speed of light, I would go back home and find that more time has passed. A year, 5 years, I don't know the math.
Wormholes allow time travel because they offer a shortcut through spacetime itself. Drag one end of a wormhole next to a high-gravity body (like a neutron star?) and time will slow for that end.
Oh, you want to go back in time? Hmm, well I understand that a lot less. But according to Stephen Hawking it's possible. You just need a negative energy field. A big one. Which would be so unstable that you would die. It's in one of the later chapters of The Universe in a Nutshell if you want to read it for yourself.
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