Clearing device registration

The xfinity.com\authorize is not working and I cant get my SmartTV authorized - after entering the 6 digit code and signing in, the message says "Deactivation Failed, you don't have access to xfinityTV"  I can watch online on my computer and the xfinity stream on my phone, so I know its working.  Online advice says to clear the device registration from the xfinity account.  I don't know how to do that.