Custom message when wordpress updates are available

Why not just remove it completely?

You can add this to your themes functions.php file:

// Hide update notice for everyone except admins
function wps_hide_update_notice() {
            if ( !current_user_can( 'manage_options' ) ) {
                remove_action( 'admin_notices', 'update_nag', 3 );
            }
        }

add_action('admin_menu','wps_hide_update_notice');