We already know how to time travel to the future, we just can't feasibly do it yet. It just involves going really really really fast (99% speed of light) and you travel forward in time! You can also stand next to a really really really heavy object and you'll travel through time slower.
Traveling to the past is another story. People thought you could do it through wormholes but if you could travel to the past then you could create a paradox. The universe doesn't like that so it does every thing it can to stop you from traveling to the past. If a wormhole was opened up that was large enough for you to travel through, it would instantly create a feedback radiation loop that would instantly destroy. That's the universes way of not letting you create paradoxes.
