In the Governing Board or Trust Board tab (this is usually the 4th tab on your board's page) select Members.
Click on the person's name to view their appointments underneath their name. Click on the three dots in the top corner of the appointment you want to update.
Click on Edit information.
This will bring up a new window which allows you to edit the member type, update terms of office, allocate a nominating authority and keep a note of any additional info.
Just click Update to finish.