How can I add a sub menu to an existing navigation menu in WordPress?

I got the answer, here it is: I want to add submenus ffrom database to the menu named ‘Products’

Create a custom plugin and install it through Admin panel. Inside the functions.php write this code. This is upgrade safe way and will not brake if the theme is updated.

add_filter( 'wp_nav_menu_objects', 'ravs_add_menu_parent_class' );
 function ravs_add_menu_parent_class( $items ) {
  $results="SOME SQL QUERY";
  foreach ( $items as $item ) {
    $title = $item->title;
    $ID = $item->ID;
    if($title=='Products')
        $parentId = $ID;
  }
  foreach ( $results as $result ) {
  $name = $result->name;
  $id = $result->id;
  $link = array (
        'title'            => $name,
        'menu_item_parent' => $parentId,
        'ID'               => $id,
        'db_id'            => $id,
        'url'              => 'URL'
    );

  $items[] = (object) $link;
  }
  return $items;   
}