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👉 Sign in to your Apple ID account page and edit / update your Apple ID details.
On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.
See also the following Apple articles:
Change, add or remove Apple ID payment methods
Change, add or remove Apple ID payment methods – Apple Support (UK)
Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)