My client has a new card and I need to update their membership
If your client has a brand new bank account or wants to switch cards completely, you need to click on the Services tab in your account and find the client.
Once you've opened the client's membership details, you'll see the option of payment methods - click on the 
sign to add the card details. After you've entered the card details, click on Add card button to save it.
Make sure to set it as default afterward, by clicking on the three dots.
Alternatively, you can watch the attached tutorial below