There are two ways to cancel selected nights of a reservation in FrontDesk Master.

Bed View

Select nights you want to cancel by clicking them on Bed View. Then, right click on the reservation and choose option Cancel Selected Nights:

Reservation Form

Open the Reservation Form and change the dates of the reservation for each guest separately or for the whole booking:

You will see all cancelled nights in the Reservation Form. You can restore them by clicking on Restore icon:

See also:

How to restore cancelled reservation

How to move reservations on Bed View