@bethany-siu - you can always just change the terms, but if you use the reservation number that contains the property information in other ways, that can sometimes mess you up - so you can always copy the reservation to a new one and then cancel and/or delete the previous one. That will give you a new reservation and a new reservation # as well. That's how I prefer to do it.
If payments have already been made, it gets trickier - you can't delete the reservation, but you can cancel it without refund, and then record a manual refund on the old reservation (don't refund card) and then record payments made on their due dates on the new reservation. If the security deposit has already been paid, then I'd leave that on the old reservation and cancel it later.
I know that sounds complicated, but it's a good work-around if you need that reservation number to be consistent with the property.