If the customer would like a replacement item sent out it depends on what historical detail you want to keep for reporting purposes on how you will handle this exchange.
If you want to keep a full history or the exchange you can clone the original sales order to a sales credit, receive the item back into stock then create a new sales order for shipping out the replacement. The credit and new invoice can be matched in order to clear the customer account of any debt and mark them both as paid.
The benefit of this method is that you can use sales credit statuses to record the reason for the credit and you can create a new goods-out note for shipping purposes, you can also easily see if a customer has been buying and returning many orders. The inventory will be moved into quarantine before begin returned to stock so you have the opportunity to decide whether it is in a saleable state before updating availability across your sales channels.
History not required
If you don't need a full history on the customer account, or you're happy to do this with notes, a simple like-for-like exchange can be done with no extra work required in Brightpearl. However, the returned item might not always be a goods as new, you will therefore need to consider the changes to your inventory value and level. inventory corrections can be done to remove the replacement item from stock at cost and a correction done to add the returned item at cost.