You can do a full or partial refund for any transaction in the Control Panel.
Find the transaction receipt. Either from their member detail history or from the payment report.
On there receipt page hit the 'refund' button.
A modal will pop up. Either leave the full amount to be refunded or adjust to a lesser refund amount.
Hit 'Confirm'
Note
Stripe will also refund you the transaction fee from the original purchase. They are so nice!
*For information on how to process ACH refunds, go here.