How to replace a wireless receiver for Uverse tv

One of our two wireless receivers is failing.  It turns on and briefly brings up channels, then the channel disappears and the tv soon says that service has been lost.  I have restarted, repowered, etc the receiver to no avail.  I switched the two receivers  and the problem follows the wireless receiver, so it is not the tv.  How do I order another wireless receiver and return the defective one?  I have looked at the website in detail and cannot find a path to do so.