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.