How to change iCloud default payment method?

The default payment method is primarily your personal account balance. After that it starts charging your alternative payment methods in the order they are presented. How apps, content, and subscriptions from Apple are billed - Apple Support

This document explains how to add a payment method to your Apple ID on a Mac, Apple mobile devices, or PC. Click here --> Add a payment method to your Apple ID - Apple Support

To find out how to remove a payment method, and what to do if you can't remove your last payment method or use no payment method with your Apple ID, read this article --> Remove a payment method from your Apple ID - Apple Support

Ongoing subscriptions and Family Sharing organizers must always have a payment method present. To edit the cards used for those you may need to first add the new card and make it the primary payment method before removing the old card (see: If you need to change or update your Apple ID payment method - Apple Support ).