In Pabbly Subscription Billing, there are two ways to add a new card for a customer -
1) Through Merchant account - In the customer's section, on a record for a specific customer, go to the bottom of the page, you will get an option to Add New Card through your Pabbly account. For instance, please check below -
2) From Client Portal - The customer can themselves update the card details via the client portal. For instance, please check below -
Also, the card can be updated for a single subscription from the portal itself. Hence, in the above two ways, the card details can be updated for a customer in the application.
For more info please follow this doc -
https://www.pabbly.com/subscriptions/docs/features/#section17