Just keep it. What I would try to do, is go there in the morning and tell the restaurant that there will only be two of you, that way maybe they can change things around if they need to. Or if you won't be at MK in the morning, maybe have the concierge desk at your hotel see if they have a direct line to the restaurant to call them. Do not go through Disney dining. If you can't call or go to the restaurant directly, I wouldn't change anything.
You can't change reservations with the current system. You can only cancel and make a new one with the correct number. But since 6 to 2 is a big jump, if it were me, I'd at least try to give them a heads up in the morning.