To process an exchange (a return and purchase) from the point of sale follow the below instructions.
From the Point of Sale interface utilize the Customer button to select the customer you are processing the return / exchange / refund to.
Utilize the transaction button to load your recent transactions.
Utilize the search field to search for the invoice you wish to return from.
Click the Return / Refund / Exchange button.
Toggle the item on the invoice that is being return then click the check button
You will now see the return added to the transaction.
Utilize the Scan or Stock buttons to search and add the item(s) the customer will be purchasing.
If the sum of the exchange results in a credit, you will have a Refund button where you can process the remainder as a refund or store credit.
If the sum of the exchange results in a balance, you will have a Charge button where you can process a charge for the remaining balance.
Please review this related article on processing returns and refunds.