From AT&T's website https://www.att.com/esupport/article.jsp?sid=KB111096&cv=820#fbid=VkgCstpCRd2
We're sorry, but because we begin processing your order as soon as we receive it, you can't change or cancel the order. However, after placing your order online or over the phone, you can:
Refuse the delivery of your shipment.
Return the purchase using the return label and instructions that are included in your welcome kit.
Let me know if you successfully canceled your order at AT&T, and if you initially bought it using their NEXT program. Those folks seem to be able to cancel at will while 2yr upgrade peeps cannot cancel.
I just cancelled the preorder through one of their representatives. I was chatting with one.