Creating a submenu in navigation

I created a responsive navigation menu with a tutorial in Hongkiat. https://www.hongkiat.com/blog/responsive-web-nav/
In that tutorial there is no sub menu items. So I want to add a submenu.

This is the demo I created in JSfiddle. https://jsfiddle.net/viveknath322/3AHTg/1/
In that demo I want to add submenu in Tools menu.
Can anyone help me to add submenu?
Thanks.
Regards,
Vivek Nath.