A return means two things need to happen: the copy goes back into stock, and the recorded sale is corrected. The right path depends on how the item was sold.
Sold through a website order
Handle it on the order itself: open the order in Manage Orders and cancel or refund it there. When you cancel, BookRouter offers to reverse the card payment and return the items to stock in one step. See Managing orders.
Sold at the Point of Sale or on an invoice
Open View Sales, find the receipt or invoice, and use Refund. A full refund reverses the payment (through your card gateway when it was a card sale), removes the sale from your history, and restocks the inventory lines automatically. See View Sales: receipts, invoices & refunds.
Sold manually (the Sell Item button)
Open the item and go to its Sales History tab. Delete the sale row and choose the option to restore the quantity. The stock comes back, the sale disappears from your reports, and the status returns to For Sale if the item had sold out.
Marketplace sales
Process the refund on the marketplace itself (Amazon, eBay, and the others handle their own payments). Then bring the copy back into stock by raising the item's quantity and, if needed, setting its status back to For Sale.
Why not just bump the quantity? Raising the number alone puts the copy back on the shelf, but the original sale stays in your sales history, so your revenue and margin reports would still count it. Correcting the sale record at the source keeps the books honest.