Cole o código abaixo no arquivo “functions.php” do tema:

add_action( 'pre_get_posts', 'my_change_sort_order');
function my_change_sort_order($query){
if(is_archive() || is_search()):
//If you wanted it for the archive of a custom post type use: is_post_type_archive( $post_type )
//Set the order ASC or DESC
$query->set( 'order', 'ASC' );
//Set the orderby
$query->set( 'orderby', 'title' );
endif;
};

😉