Click on any Task Event event in the Diary or Scheduler tabs to bring up the Actions Panel. From here, choose any of the options to do more with the Task Event.
Mark As Complete
Click on the tick to mark a Task as having been completed
Open up the Task to edit the detail of the task itself
Delete the task completely.
You can also drag & drop tasks/diary events wherever you need them - though you cannot drag something from one campaign or dashboard to another.
If a diary entry is an "All Day" event, you can click and hold the right border to drag it across multiple days.