Modifying a single menu link title
If you only want to change the title of an menu item, you should be able to do this through the administration. Go to admin/structure/menu and edit the correct item in your secondary menu.

To use hook_menu_alter() you must add it in a custom module not inside your template file. Remember to clear you cache in order for the changes to take effect. To clear the cache you can goto admin/config/development/performance.