There are 3 methods to assign a reminder as completed:
METHOD 1
You can check it as completed from the pop-up window that appears when you log in and the reminder is due. Mark the "Do not show reminder again" button.
METHOD 2
Click the "View all" on the purple calendar to open your reminder list.
Select the "Save and Exit" box to check a reminder as completed.
METHOD 3
You can as completed from the CRM module, under the listing there is a small section called "Programmed Reminders". To mark a reminder as completed, click to mark as YES.
NOTE: Here only the reminders of the day will appear. You can navigate forward and backward.