Updating a reservation on Booking.com must be done by the guest. The guest needs to request a reservation update on the reservation details page and click on "Edit guest details":
In this section, the guest will be able to update dates / guest information. In order to update the booking dates, the guest can click on "Change dates":
If the property is available for the new requested dates, this will directly update the reservation dates:
Note: If you want to cancel a reservation from Booking.com, it needs to be done in the listing site itself.