As an admin, you can't. Changing reminders for a Calendar Event can only be done by the constituent when he or she RSVPs for the event or by going through the RSVP process again to update the information.

If the constituent would like to change their reminder date after they have RSVPed, have them go through the RSVP process again. The RSVP process will appear to re-RSVP them. It does not. Instead, it updates their RSVP information.

Note: If you turn on the Event Reminder in the middle of the Event, the "Would you like to receive an email reminder for this event?" will only appear to the new registrations. All other registrants prior to when the reminder was activated will not be sent a reminder. It will only send out the reminder to those people who selected "Yes" after the reminder was turned on.