change credit card for existing iCloud account

Please clarify how this happened.

Do you have her card on your Apple ID?

Or are you part of a Family Sharing group where she is the Family Organizer?

For the first one, remove the card from your account.

For the second one, you can't. In Family Sharing, the Family Organizer pays for all purchases of all Family Members. There's no way for you to use your own payment methods if she is the Family Organizer and purchase sharing is turned on.

Set up Family Sharing - Apple Support

One adult in the family — the family organizer — can set up Family Sharing for the group from their iPhone, iPad, iPod touch, or Mac. If purchase sharing is turned on, the family organizer pays for family members' purchases and must have a valid payment method on file.3

You can add Apple, iTunes and App Store gift cards to your account, and it will use that balance before charging her card.

You're welcome.

As I said, you can add gift card credit balance to your own account and it will use that before charging her card.