Hi, I am Rob and i will do my best to help you.
what site are you trying to change the number on please ?
Navigate to the Barnes and Noble home page and click the "Sign in" option on the top row. Enter your username and password ***** then click "Secure Sign In."
Click "Manage Default Payment" under the Payment section.
There should be a way to add a new credit card / delete the old / make whichever one you want to use the default
Click sign in on the top left
Click my account
Then manage account
Then payment settings
There you can add and remove credit cards and set the default if you have more than one