They should take anything back within time limit stated on the back of the reciept. That meets the qualifications listed there. So within 14 or 30 days almost any item complete with its original packaging, and reciept should be accepted for full refund (even if open), NO QUESTIONS ASKED. Open laptops, digital cameras and camcorders are subject to an open box fee, and must be defective, and open cd's, videos, dvds, or software is only exchanged.
Its all clearly stated on the back of the reciept.
If they give you trouble, ask to see a manager.
Try not to say you didn't like the item. Say something like you thought it did somthing that apparently it doens't, or you bought the wrong one by mistake and opened it before you realized it wasn't the right item. If you make it sound like you have the intention of purchasing the correct item now, i.e. the money isn't really leaving the store, they should be real nice. If they try and get you to exchange on the spot, tell them you want the refund and purchase as seperate transactions, due to your credit card policy or that you want to use a different card or pay cash.