Where do you pay ICloud

If you have an iCloud+ Plan, it will be paid from your Apple ID's payment method.

If you want to change your payment method, you can do it on your iPhone here:

Settings → Apple ID → Media & Purchases → View Account → Manage Payment

Select Add Payment Method. Once your new payment method has been added, you can delete the old one.

You can find more useful information here:

Add a payment method to your Apple ID - Apple Support