Once in the customer's "Account Data" you will select the "Payments" Tab.
The image above is what you will see once the Payments Tab is selected.

(RED): This is all of the Payments taken on the customers account.

(ORANGE): You can view the details of each charge -  payment via, the payment amount, paid by, and reference number.

(GREEN): Under the "Actions" section you will see the "Refund" button. Once selected you will enter the amount you would like refunded.

Once refunded you can then select the Reference Number to view the closed out job and see the details there as well. (refer to the image below highlighted in RED). Refunds will display as a negative charge.