Unfortunately during the course of your online sales you will probably run across the need to process refunds.
Step 1 - Locate what payment method the customer used
First identify if the customer payed with Paypal or Credit Card (via Authorize.net). You will then need to login to the respective site handling the transaction (either Paypal or Authorize.net) Once logged in you can search for the customer via Address, Last Name, Email etc.. Verify that the customer is the correct one by checking total price and address details.
Step 2 - Refund Order in Paypal or Authorize.net then mark order as refunded in Cart Ninja
In both Paypal and Authorize.net you can process a partial refund or a full refund. Once the refund is completed you can go back to the Order Management in Cart Ninja and mark the order as refunded