How to delete a credit card that is not valid?
Go to Zoho Books - Sales - Customers - search for a customer - Overview - Credit Cards - Delete.
If you need to check the transactions to see which one should stay:
Go to Zoho Books - Sales - Customers - search for a customer - Transactions - Open the most recent invoice - Payments received - Payment mode - look at the last 4 digits of the credit card used.
Delete the old/not valid card