If it's defective, you take it back to the store. Assuming you're within the warranty, they'll swap it out for a new ...whatever. If the warranty was purchased within the last year, you can only return an item once. There are some older warranties that allowed you to swap an infinite amount of times within the period allotted, but I think they may be expired by now.
You should have the receipt, aka proof of warranty, when you go in, but if you don't, call ahead to the store and tell them what's going on. A copy should be on file somewhere, and they should be able to dig it up.