This help article will show you how to refund a purchase to credit.
Find the invoice that you would like to refund
Select Refund
Select Refund Payment and Item or Refund Payment
If refunding Payment and Item - Enter the quantity that you want to refund in the Count at the top right hand side
Select Credits
Select Refund
If you click on the member's name, you'll be dropped into their On Account area where you'll be able to see their Credit Balance