If you purchased it from ATT then you have a 30 day return that you can use, change plans to any other ATT phone, and thats it.
If you purchased it from Apple, return the phone then go to ATT and explain the situation and they will allow you to "upgrade" to another phone.
Use the money from the return to buy a new phone.
I have just done the same, and I had my iPhone for 3 months.