"I høsten" means as The previous poster said, "in the fall/autumn" however, as you may notice, this sounds a bit awkward, which it does in Norwegian too. Can't say I've ever heard it used. However, "til høsten" is a lot harder to explain. Briefly explained, it may mean that someone is talking about the next autumn or something happening up until the next autumn. Both usually involve an action.

"I høsten" means in autumn/fall and "til høsten" means to autumn/fall.