To change it: Log out of Workgroup Manager, then log back in as the System Administrator (root).
Tip: By default, the root user has the same password as the primary administrator account that was originally created in the Server Setup Assistant.
You should now be able to edit the primary administrator's password. After doing so, be sure to log out the root user and log back in with your regular administrator account.
Note: Be sure the search path is set to "Local", as the record for the Administrator resides in the local NetInfo database.