Hiding sun nav item

Hi,

I would like to hide all the sub tabs except “Email” when Settings in main nav is clicked. Please refer to the screenshot. How can I achieve this?

https://drive.google.com/file/d/1HZGpi3buplInhmai4QPwV1CJcJF0GD1_/view?usp=sharing

The page I need help with: [log in to see the link]