Modify Cancel Participation Data - Reason Cancelled

For a participant booked into a classroom and then needs to be cancelled. In Cancel Participation: Data screen, if a "Cancellation Reason" is selected and later needs to be modified to a different reason, how do you make the correction to a different reason?

ie Initially selected "15 No Show" and need to modify to "2 Away From Work."

Please advise.

Regards, Pearl