How a Customer can Update their Credit Card Details for a Membership
The setting to allow customers to update their payment details must be enabled by navigating to Settings > Global Configuration and ensuring ALLOW CHANGE PAYMENT = Yes
Customer must log in to the Customer Portal:
How Customers can Create an Online Account Portal
If the customer has no payment details recorded, they will immediately be prompted to update their payment details (press Update Now will take them to the Manage Order screen):
If payment details already exist but the customer wants to update them, they can log in, scroll down, find the membership they wish to update, and click "View Order" next to it.
They will then be able to Manage Order and add new credit card details or update existing ones, if needed.