On the account, I don't remember if it's under "my profile", "payment method" or "my plans", "manage" and "payment method", if you reach the maximum number of credit cards allowed, you can contact customer care to help you.
I think it's under "my profile". It's also there you can change the default paiment method if you have more than one card. It's not there when you don't have reach the limit, so I can't check. But I saw this link few days ago.
Otherwise, chat with customer care, they will fix for you.