1. In Records, click Events.
    2. Click Open an Event and search for and open the appropriate event.
    3. Select the Participants tab
    4. Select Guests in the left frame
    5. In the right frame, double-click the guest to open the participant record
    6. From the menu bar, select Participant, Convert to Registrant. 
    7. Save and Close the participant record. 
    Convert guest to registrant

    Alternative solution for prior versions: 

     

    1. Delete the guest
    2. Recreate the participant as a registrant.