where do I find iCloud with 50 GB of Storage Monthly charges setup to change it?

Just found the answer:

Follow the instructions below to change the iCloud payment method on your Apple ID account from any device.

On an iPhone, iPad, or iPod touch:

1. Go to Settings > [Your Name] > Payment & Shipping.
   1. On iOS 12 or earlier: Go to Settings > [Your Name] > iTunes & App Store. Then tap your account and select View Apple ID > Manage Payment.
2. If prompted, enter your Apple ID details to sign in.
3. Tap Add Payment Method and fill in the necessary details to add a new payment method to your account.
4. Tap an existing payment method to update the details for it. You may need to re-enter the full details when you do this.
5. Finally, tap Edit to remove or reorder your payment methods. Remember to move your preferred iCloud payment method to the top of the list. (It’s not possible to move payment methods above your Apple ID Balance.)