In the Governing Board or Trust Board tab (this is usually the 4th tab on your board's page) select Members.
Find the person on the Members list and click on the arrow to the right-hand side of their name to reveal a drop-down menu.
To update or change the term of office dates, click on the pen icon on the smaller box under the main profile.
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.