There are three ways to change or remove working hours / schedules from the calendar.
Delete / change a single working day via the calendar
In the calendar, click the 3 dot menu at the top of the staff column
Select "Edit working hours"
Edit the working hours or uncheck the "Working" checkbox and click "Save"
Delete / change working days via the schedule app week view
You can edit multiple days for multiple staff all on the same screen with this method.
Go to the Schedules app via the right hand menu
Click on a week in the planner to open the week view
If you click on a shift, click on "Edit schedules"
Edit the working hours or uncheck the "Working" checkbox.
Click "Save" at the bottom of the screen.
Delete large numbers of working days using Delete schedules
Go to the Schedules app via the right hand menu
Click on the red "Delete schedules" button at the top of the screen
Select the details of the staff and dates of schedules to delete
As you make your selections, the preview panel will display details of shifts to be deleted and any appointments that will be left unstaffed.
Click on "View list" to see details of these appointments.
When ready, click the red "Delete" button and confirm.
Important
Appointments will never be deleted by Collums. If you remove a schedule leaving an appointment unstaffed, it will still appear in the calendar. The staff column will disappear once all events have been removed from it.