I'm Rahul Kumar Singh, from the Edwiser team.
Since you are using Edwiser RemUI v3.5, Inorder to change the icons on the left navigation menu, please follow this path -
Go to the theme file --> Open 'Lib.php' --> Search for function 'flatnav_icon_support' --> Once there, on line 126, you can find the option for adding/changing the icon for 'Courses' --> Just replace the 'fa-archive' with the icon code you want from the fontawesome icons repository.
Note: You can only use icons from fontawesome repository, as custom icons are not supported by Edwiser RemUI yet.
I hope this comment solves your query. If you have any other queries, you can contact us at Edwiser@wisdmlabs.com for a quicker response.