iCloud 
Mark a reminder as completed or not completed
A reminder stays in the reminder list until you mark it as complete, at which point it moves to the Completed folder within the list.
Mark as completed
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Select the list that contains the reminder, then click the circle to the left of the reminder.
The reminder moves to the Completed folder.
Mark an item as not completed
Select the list that contains the reminder, click Completed, then click the circle to the left of the reminder to deselect it.