To disable a planned maintenance you need to :
click on "Equipment" on the banner on the left of the screen
click on the equipment on which you want to deactivate maintenance
select the maintenance to be deactivated
click on "Edit" on the right side of the screen
click on "Disable"
You can easily deactivate a maintenance, however it is not possible to deactivate the first maintenance done in order to avoid possible falsification of maintenance history.