Sign in into the Text Integration website: https://secure.textintegration.com/
Select My Account and then Credit Cards.
To the left you will see a list of credit cards that we have on file. You are able to add a credit card by clicking "Add credit card".
Kindly use the ADD credit card button at the top and NOT the update card button on the side. The update button will only allow you to change the name and the billing information of the card and not the card number itself.
If you are having difficulty adding a credit card information into your account, kindly check this article, Having difficulty adding a credit card?