Hi @Anonymous User
Any error message on screen when attempting to change your bank details or does the page constantly loads.
I would try a different internet browser with the ad blocker disabled or use your browser in a private window mode when updating your account details.
I am assuming you are using a UK registered Bank ?
If no joy, see if nowtv live chat can assist you further.
Try this link for live chat.
https://help.nowtv.com/contact-us/managing-my-devices
