How do i change the credit card being charged monthly?

iCloud will use the payment methods added to your Apple ID, in the order they appear in. If you need to add a new payment method, and then make it the default for all payments, then follow the steps in the support document linked below.

click here ➜ Add a payment method to your Apple ID - Apple Support

You can then drag the payment methods on your Apple ID up or down, to set a specific order.

Payments, including iCloud storage will try to use the first payment method listed, and then move down the list, if for some reason it cannot be charged.