Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
I'm not quite clear if you're wanting to change your default card for all purchases, or specific to your kindle.
In either case, this is best done from your computer on amazon.com.
First, go here:
there you can be sure your card/bank info is correct (right address, valid expiration date, etc)
If you're only wanting to change your default card for prime purchases, that can then be done from here:
From there, go here if you are wanting to update your kindle account info:
On this page, click on the SETTINGS tab at the top right,
then you can from there set a default card to be used for one click purchases on the kindle.
Under Kindle Payment Settings, click on the Edit Payment Method button, and you can set the card you want used for your kindle purchases.
After that, reboot the kindle by pressing/sliding and holding the power button for 20+ seconds, and it should operate normally from there.