How to Change Menu Icons
In order to change menu icons, please navigate to Appearance ->Menus -> Main menu.
Click on Screen options section in the top right corner to make sure that CSS classes property is enabled:
Then you will be able to replace the icon css class for each menu item:
The list of available theme icons can be checked opening micro-office\css\fontello\demo.html file or following this demo link: http://microoffice.themerex.net/wp-content/themes/micro-office/css/fontello/demo.html
In case you want to add new icons to the theme fontello list, please follow this video tutorial: