Hi and welcome to the forum @Anonymous User,
As you may or may not be aware, the NOW TV service permits you to register up-to four playback devices and view their service on up-to two at the same time. You can find out which devices you currently have registered by selecting the: 'My Account' button and accessing 'Manage Devices'.
Ordinarily you are permitted to change one registered device per month. However, for 'forced' device limit resetting you will need contact a NOW TV representative, which can be done via the forums 'Live Chat' facility or 'Send us a message' facility (but please be aware they are not obliged to perform this service). Here's what you need to do: -