I want to trade my iPhone 4S?

So I got my iPhone July 29th, and today is August 27th, and so after a few weeks of
having this phone, I've come to realization that I DO NOT like this phone whatsoever. I want to switch back to Android and get a Samsung Galaxy S3, and I know it hasn't been 30 days yet so I can still switch it out. But my question is how? And how much will it cost me?

Interested in an ATT 16GB Black Lumia 900?