How I can use order by of the custom post title?

You can use the order and orderby param in WP_Query argument like below:

$args = array(
        'post_type' => 'case',
        'post_status' => 'publish',
        'meta_key'  => 'client',
        'posts_per_page' => 9,
        'orderby' => 'title',
        'order'   => 'ASC',
);
$query = new WP_Query( $args );

It will list out your posts by title. For more information check official WordPress document