To remove a credit, you would pull up the customer's account and then go to billing Information > Payment Accounts
In the payment accounts window you'll see an adjust credit button.
When you click on that button, the credit adjustment window will appear.
Enter the change in credit you would like to apply. If you are trying to remove a credit enter the amount as a negative amount. After you have entered the amount, hit process. If you are trying to process a negative amount to remove a credit, the system will ask you to confirm before processing the change.
To add a credit, you will just not put a negative before the credit amount. A credit to the account will be represented like shown below.
If the customer has an outstanding balance that exceeds the credit, you will see a triangle with an exclamation point in it by the amount. Put your cursor on that symbol and let it hover to get more details. The screen shot below shows a situation where there is an account credit of $20 but they have an outstanding invoice of $99.
