vertical menu bar

You can either use a filter in your functions.php file:

function mytheme_nav_menu_css_class($classes, $item, $args) {
  if($args->theme_location == 'secondary') {
    $classes[] = 'your-class-name';
  }
  return $classes;
}
add_filter('nav_menu_css_class','mytheme_nav_menu_css_class');

Or if you go to your WP Admin, click Appearance -> Menus and then click the tab at the top “Screen Options”, you can tick CSS Classes which will enable you to add a class to each list item in your menu.

See this article for details.