Click on "Create Reminder" in the top right corner of the screen.
Select a recipient of the reminder from your contacts:
Fill out the date and time, then type in the message. When you are finished, click on "Save"
To update a reminder, navigate to the calendar.
Click on a date with a blue circle next to it. The blue circle indicates that a reminder is set there.
Once you click on the date, it will show you the reminder that is scheduled.
Click on the reminder to update it. The only thing you cannot modify is the recipient.
You could also delete the reminder from here: