From the Sales Screen
You can create a return directly from the Sales screen simply by clicking on the Sale button and changing it to Return:
You will then complete this as you would a normal transaction.
From An Existing Transaction
You can create a return from an existing transaction by pulling up the receipt. You can do this by looking under the Customer's transaction history:
Or by simply scanning the barcode or typing in the POS number on the receipt under Sales > ... > Lookup Receipt:
Once this receipt has been pulled up, you can simply select Create Return: