Credit card change for buying iCloud

Here are support documents with useful information:

Change or remove your payment information from your iTunes Store account (Apple ID) - https://support.apple.com/HT201266

Payment methods you can use in the iTunes Store, App Store, and iBooks Store - https://support.apple.com/HT202631

If your payment method is declined in the App Store or iTunes Store [or you have a billing issue with a previous purchase in the iTunes Store] - https://support.apple.com/HT203005

If you can't remove your last payment method or use no payment method with your Apple ID - "Learn why you can't use your Apple ID without a payment method in the App Store, iTunes Store, and more." - https://support.apple.com/HT203905

#1 in the last link is a frequent cause, where somebody has a subscription which means you cannot change the payment method. You must wait for the subscriptions to expire, or contact Apple and see if they can resolve the issue.

This is the most direct method to contact Apple for media/app/iCloud store support:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->

https://www.apple.com/emea/support/itunes/contact.html

Use this method to get telephone support:

Contact Apple for support and service - "See a list of Apple phone numbers around the world."

https://support.apple.com/HT201232