I would just leave the reservation as is and when you arrive at the restaurant, let the CMs at the podium know that there are only x number in your party. As long as at least one person shows up for the reservation, you will not be charged for the missing person.
You will have to call the dining line to modify the reservations, but the only ones I would modify are any pre-paid meals (e.g. dinner shows, CRT, Wishes Dessert Party). Be sure you let the CM know you don't want to cancel the reservation, just get a refund for one person.