Whenever possible, returns should be accompanied by the original sales receipt.  When you have that, you can enter the receipt number into the POS and that sale will come up.  You can then tick off whichever item is being returned, and Square will automatically calculate the correct amount to return (accounting for discounts, etc), which makes the process much easier.  If the customer doesn't have their receipt, you can still search for their sale by using the last four digits of the charge/debit card (if the sale was processed as charge/debit), by the date that the sale took place if the customer remembers it, or by their name if you entered it when the sale was rung up.  You can also search for sales that included the item that's being returned in the Dashboard and hopefully find the sale that way.  If none of that works, you'll probably need to return the item outside of Square and manually enter it into whatever inventory software you use.