1. Go to Tickets > Program Search.  Search for and select your program.
  2. Go to the Events List tab.  Find the event that person was registered for.
  3. Click on the event, then click Edit event in the top left corner.
  4. Increase the capacity of the event by the number of tickets refunded.  Click Save.
  5. This will allow you to sell those tickets, but note that the original registrants will still be on the event and will not be marked as cancelled.