If someone accidentally purchased too many tickets or needed to adjust the number of tickets purchased, the easiest way to handle that is just to delete the transaction and place it again.
To delete a purchase for a bidder, find the bidder record and open up their detail (click on their name). Then click on the Purchases tab.
Find the purchase in the list of purchases and click on the "X" next to it. Confirm the delete and then go to the invoice to confirm that the deletion has updated their invoice properly.
If that was their only purchase, then they will now have a $0 invoice. If they have already paid for the item their invoice will show an overpayment. Please contact the service team (firstname.lastname@example.org) if you need to issue a credit card refund.