How and where do I update my credit card info for iCloud storage

iCloud uses the payment methods assigned to your Apple ID.

Follow the steps here to change it: Change, add, or remove Apple ID payment methods - Apple Support

If you are using a different Apple ID for iCloud , that the one you are using for the iTunes & App stores for some reason, then you will need to sign in to that Apple ID on iTunes in a computer or through the https://tv.apple.com website to add the payment method as explained above

Via - Change, add, or remove Apple ID payment methods - Apple Support.