Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect from the sound of it, that you need to remove the credit card number currently registered in your Amazon.com account, after logging in to your Amazon.com account.
Then, you can enter your new credit card number from scratch, into your Amazon.com account, while still logged into it.
This is owing to the currently edited credit card number not being editable or capable of being changed, except for its expiration date of course.
Let me know how trying that goes, and mention any problems or questions along the way, please.... using the Reply To Expert function. Or if my suggestion works, I'd appreciate hearing about it! Thanks.