change payment for iCloud plan

You cannot set up a payment method specific to iCloud, it is set up as a an automatic secondary payment method after your account balance for anything you buy from the iTunes and Media stores. From --> How apps, content, and subscriptions from Apple are billed - Apple Support

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

Some purchases aren’t charged to your Apple ID balance

● If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method.

● You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

● Some subscriptions might not be charged to your Apple ID balance.

If none of your payment methods can be charged

If a purchase can't be charged to any of your payment methods, your account has a negative balance. You can't buy more items or update your apps until you update your payment method."

This document explains how to add a payment method to your Apple ID on a Mac, Apple mobile devices, PC, or online. 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 ).