If you know a staff member can no longer work a shift, you can take them off the shift and republish an open shift:
From the week or the day view, hover over the shift you want covered and click the icon. Or, you can click on the shift to view details, and click the same symbol in the shift view.
Click 'Yes, cover shift' The shift is now published as an open shift.
The shift appears in red, denoting that it's been published to available employees.