Having a well-thought-out return policy is key to attract - and keep - your customers. Making it easy for you to accept and refund those returns is therefore also a key aspect of your Point of Sale interface.
From your Point of Sale interface, select the product your customer wants to return, use the +/- button and enter the quantity they need to return. If they need to return multiple products, repeat the process.
As you can see, the total is in negative, to end the refund you simply have to process the payment.