Try contacting the guest and apologising and explain and ask to cancel. You cannot do the cancellation yourself. Maybe do some research so you can offer some alternatives and they are more likely to cancel. If you ask booking.com to do it they will charge a Relocation fee
Extranet
Reservations
Reservations Details Page
Right Pane, Request To Cancel
Choose option 2, submit
Wait for guest to receive email with action to confirm
Try contacting the guest and apologising and explain and ask to cancel. You cannot do the cancellation yourself. Maybe do some research so you can offer some alternatives and they are more likely to cancel. If you ask booking.com to do it they will charge a Relocation fee