It should be very easy as they have a 30 day return / exchange policy that is printed on your receipt. Here is the relevant section:
Returns/exchanges must be like new, with all original packaging, accessories, manuals and proof of purchase. You may exchange equipment one time within the 30 day period.
iPhone returns/exchanges will be subject to a 10% restocking fee, except where prohibited. A $20 restocking fee applies to devices purchased at an AT&T-owned retail store except where prohibited. Restocking requirements and fees may apply to purchases made elsewhere, and may vary by market or type of equipment. See your receipt or an AT&T store for details. Equipment received through an insurance claim cannot be returned or exchanged under this policy.
Source:
http://www.wireless.att.com/cell-phone-service/legal/return-policy.jsp
Dave