Board admins have a different set of permissions on a board and control all of the settings on a board. Only admins can close a board or change the board title, for instance. Only board admins can make another board member an admin and all boards must have at least one admin.
Changing an existing member to an admin
To make a board member an admin, click the ‘Share’ button from the top of the board, then select the drop-down next to their name and avatar and select ‘Admin’.