There are 3 methods to assign a reminder as completed:
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.
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.
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.