You can cancel from the reservation itself. Whether it costs you anything depends on one thing: how close to the appointment you are.
Open the reservation
From the Reservations tab, tap the booking you want to cancel. The detail opens over the list.
Tap Cancel
The button is in the detail. It is there while the reservation is waiting for confirmation or confirmed, and gone once the service has started.
More than three hours ahead: just confirm
Cancel that early and there is no charge and no warning. Confirm and the reservation is cancelled.
Inside three hours: read the warning
Davinote warns that you are inside the penalty window and shows exactly what the charge will be. Read that figure rather than assuming one: it depends on the booking and on the currency it is priced in. Nothing is charged until you accept.
Choosing the reject option, or dismissing the dialog by tapping outside it, cancels nothing and charges nothing.
Confirm the charge
Accept and the reservation is cancelled and the penalty is charged. The cancellation happens first, so it always goes through.
Check the history
The cancelled reservation moves out of the active list into your history, under the month it was booked for.