If your client needs to update their credit card information, you can guide them to do it themselves with this article:
https://recesstv.zendesk.com/hc/en-gb/articles/1500012877321-How-do-I-add-or-update-my-payment-information-
Or you can get into their account and do it for them!
Step one:
Locate the client in your dashboard- People > Client
Step two:
Click the card on the top right of their profile
Step Three:
Add/Modify their card details and save.