When a guest makes a booking, their reservation is confirmed immediately. Sometimes, guests want to make changes to their reservation later on.
If a guest contacts you to change their booking, advise them to do this through their confirmation email or Booking.com profile. You can't make changes on their behalf.
You can only modify bookings (prices or dates) on or after the check-in date, through the Reservations tab on the Extranet:
Just click on the reservation number or room name and select an option from the menu on the right-hand side of the screen, under Update this reservation.
For info on when and how to cancel a reservation – read this article.