Q: How do I update the Bank Account to pay my iCloud Storage plan

Hey,

Hopefully this can help you:

iOS:

Go to Settings > iTunes & App Store, tap your Apple ID, and tap View Apple ID. Then tap Payment Information, and update your account. You might need to enter your Apple ID password.

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If you are not on a device running iOS check out this link which guides you through updating your iCloud payment information for each apple product.

iCloud storage upgrades and downgrades - Apple Support