In order to edit several activities at once, mark all activities you would like to change with the checkmark in on the left side of the grid. Marked activities are highlighted with a blue background.
Now right-click the in the schedule grid. Make sure to click within the blue area. Chose "Edit" from the menu. The appearing window allows you to make changes to all marked activities at once. You may select to start/mark as not started, end/mark as open, and assign a responsible user.