From the Orders page, click on 'View' button of the order you want to refund.

Partial Refund

Refund by item

Select the items you want to issue a refund for, then click 'Void / Refund line(s)' button

Refund a partial amount

Enter the amount you want to refund then click 'Void/Refund Amount' button

Total refund

Make sure the 'Enter Amount' field is empty, then click 'Void/Refund Amount' button

Confirmation of refund

For either type of refund you want to issue, you will have to confirm that you want to perform this action.