Can I exchange my phone for a different one?

I have a samsung Captivate and I've had it for about three months. The phone is complete CRAP. It freezes constantly. Unless you have a phone like this you don't know how frustrating it can get. It freezes while texting during phone calls while in apps when turning on and just at the home screen when I'm not even doing anything. I want to get rid of this phone now for another phone but my 30 day trial period is up and I don't want to pay cancellation fees is there any way I can do that. I have AT&T