If you know now what the new party size will be, you can search for a reservation that size, book it and then cancel the original one. Or you can call to reduce the size.
If you're not sure what the new party size will be, you can keep the current ADR and just let the restaurant know when you check in. There's no penalty for showing up with a smaller party than booked - as long as at least one person shows up to eat there, you won't be assessed the no-show fee.