Note: If you are attempting to refund a Stripe credit card payment you have processed through SimplePractice, read this article: Refunding a credit card payment.
If you need to issue a refund to a client, you can record that in the client's record by creating a billing adjustment.
Go to the client's Billing Details page and add a negative payment:
When you click "Add Payment" the Adjustment will be added to the client's record and the client's balance will adjust to show the refund.
This adjustment will appear on statements and superbills you create for this client.