Can I trade my iphone 4 for the 4s?

I recently got an iphone 4 like a month ago (I know I'm behind) and I was wondering if the apple store would let me trade it for the 4s. And would there be any extra fees? And if that doesn't work what else would work?

If you have an upgrade available, you can usually swap it for a newer model and only pay a few dollars.

They might let you or they might charge you a changing fee, or worst of all if you are on contract they woud make you pay the rest of contract off before you can change it. Other than that i'm not sure what to suggest