Go to the Email Template editor, copy one of the existing reminder email templates (such as Default Game Reminder or Default Other Reminder) or create one from scratch, and edit it to your liking. Click Save when you're finished.
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Note that when you're planning on using a template for event reminder emails, certain extra variables are available for you to use. They can be accessed from the "Template Variables" drop down selector on the top right of the edit window, toward the bottom of the list is the "Event Reminder Only" section.

Note: As of now, the event reminder specific variables will not be replaced with customer/event specific values in the preview mode, however, they will when being sent out to customers.

Once you're done putting together your email template, head over to the Event Type Management page, and select the event type you'd like to change the reminder for. On this page, scroll down until you see the selector for Reminder Email Template. Select the template you just created, and you're done!
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