From the History tab of your Bookings page, you'll the Unmark as no-show button on the card of the session that you unmarked a no-show.

You only have 24 hours from when you marked the other attendee a no-show to unmark the no-show. After that, the button will disappear.

If you'd like to unmark a no-show after the 24-hour unmark window has lapsed, please let the ADPList team know the name of the attendee and the session schedule of the marked no-show session you'd like to unmark.