1. Register the returned item normally as you did when selling it
2. Swipe horizontally over the previously registered item in the transaction screen (top left of the main screen)
3. A menu will become visible. Choose »Invert». Inverting means that »1 x Jeans  >  90.00» will become »-1 x Jeans  >  -90.00»
4. Now register normally the new item the customer has choosen as replacement

Closing the transaction now, Cashy-POS will show you the difference in prices (if any) of the two items in the transaction. If the replaced item is cheaper then the new one, it will display you the amount the customer should pay additionally. If it is more expensive, it will display the amount you should return to the customer