@Woz1
You can update the booking yourself by using the "change" option on the reservation.
In the change form first change the new check-in date, it will show the new calculated price.
Then click in the price-field and enter price you agreed with guest.
If OK change request will be sent to the guest.
Guest will need to confirm the change and Airbnb will charge the price-difference between the old and new reservation.