Change Completed status of an activity
When you consider that an activity is finished, you can mark it as completed.
Completed is a toggle option, which you activate and deactivate using the same menu option or shortcut key. A tick specifies that the activity is Completed.
Open the activity and uncheck the Completed icon at the top-right of the dialog.
Hold the mouse pointer over the box in the Completed column in the Activities section tab. Then click the checkbox .
Right-click the activity and choose Completed.
To edit a completed activity, you must first uncheck the Completed status.