WordPress Menu item > Sub menu Class update

The below will add a class called class-here to all menu items:

/**
 * Filters the CSS class(es) applied to a menu item's list item element.
 *
 * @param array    $classes The CSS classes that are applied to the menu item's `<li>` element.
 * @param WP_Post  $item    The current menu item.
 * @param stdClass $args    An object of wp_nav_menu() arguments.
 * @param int      $depth   Depth of menu item. Used for padding.
 *
 * @return Array $classes
 */
function theme_prefix_nav_menu_css_class( $classes, $item, $args, $dpeth ) {

    $classes[] = 'class-here';

    return $classes;

}
add_filter( 'nav_menu_css_class', 'theme_prefix_nav_menu_css_class', 10, 4 );

You could add this to your themes functions.php file. There’s a bunch of other hooks when the WordPress WP Nav Walker creates the menus – Check out The Code Reference