Custom reminders set for events can be edited or deleted as long as the event has not passed. Once the event has passed, the reminders cannot be edited or deleted.
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At the top of your dashboard, click the module access icon > Administration > Reminders > View Custom Reminders to open the Custom Reminder page.
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Custom reminders set for upcoming events are displayed. Use the filter on the right to view the reminders that were set for past events.
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Click Edit to edit the reminder settings for an event.
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Click Delete to delete the reminder settings for an event.
For more clarification, please watch this.