If you pre-ordered in a store, you will probably have to go back to that store with your receipt to cancel it. If you pre-ordered it online or over the phone, I would give AT&T sales a call to cancel it.
it actually tells you online. I think you refuse it upon delivery or accept it and not activate it then return it. That's the only way, you can't do it before. You have to wait til you get it.
If you preordered it in a brick and mortar AT&T store, you have to wait until it actually ships to the store to cancel the order. That's what the rep said when I placed my order. Not sure how it works online though.
I decided to cancel my online pre-order and do in-store instead of delivery (luckily not too many pre-orders at the particular AT&T store I went to), as I figured I would be more likely to get one on the release date.
All I had to do in my case was dial the Premier 800 number and cancel the order. They didn't give me any issues or say they couldn't do it. I received a cancellation e-mail within 15 minutes. Of course my initial order was on the 10th, so maybe that fit within the cancellation window, or Premier doesn't mess around..