Jack, Put your list on the screen, put your curser on the first event you want to edit. Key F8. This will give you a Event Record. In the lower center part of the screen is the box to mark/unmark Principal. After you have done your editing key F4 (to go down the list or F3 later to go up the list) this will save your edit and give you the next record screen from the list.
Also on this screen is a box to mark/unmark that is called Allow Non-Pricipal Event. This is the only screen that has this box. This feature allows you to enter events that will not show on a report that calls for Principal Roles.