Monthly fee for storage

Please refer to Change, add, or remove Apple ID payment methods - Apple Support.

To change your payment method - On your iPad, go to:

Settings -> Apple ID (Your Name) -> Payment & Shipping.

Select Add Payment Method, select the way you want to pay and register your new card.

You cannot delete the old payment method until you have created a new one. However, this only applies if you have some ongoing subscriptions, such as iCloud Storage or Apple One, etc.