Hello You cannot change your payment details directly on the Kindle (Amazon has changed that). You must go on a PC and log onto www.amazon.com and follow the steps below:
Amazon Device Support › Managing Your Fire & Kindle, Kindle Content, and Account›
Manage Your Kindle Payment Settings
You can view or edit your 1-Click payment method by visiting the Settings page within Manage Your Content and Devices.
To view or edit your 1-Click payment method:
If you need further assistance let me know. Please do not forget to click a POSITIVE RATING before you go- as it's how I am credited for my work. You can follow up at any time, at no additional cost.