@Karen295
I believe you have a booking request rather than in inquiry. Booking requests do not offer the "special offer" option as they are ready to book at your asking price. Keeping your calendar and pricing up to date is important - if I were the guests in question I would have gone elsewhere pronto. But if your place is the only suitable for them, maybe they are willing to accept the new conditions.
You can do as @Gerry-And-Rashid0 says, but bear in mind that they are free to decline your alteration request. A second option could be for them to cancel their request while you update your pricing, then send a new request.