How to add subtitles to menu e.g PORTFOLIO -> Our work, BLOG -> News etc...

1. Please go to WP admin -> appearance -> menus and edit Your current primary menu. 

2. Edit top level menu item that You want subtitle to be added or changed.

3. Locate option called Description inside menu item ( if it is not visible then go to top right corner of page and click on screen options tab and enable it under "Show advanced menu properties" section )

4. Once option is visislbe -> edit filed and save menu.