In the Class of Service-Admin Guide wiki (https://wiki.freepbx.org/display/FPG/Class+of+Service-Admin+Guide) I see the following:
"You can double click on an extension to move it to the other list."
This functionality seem to be broken. I tested on several computers (Windows/Ubuntu) with several browsers (Chrome/Firefox/IE).
Another problem is that you also can't drag and drop an extension when you have a lot of extensions. This is because you need to drop it on the text "Include" or "Explode", you can't simply release it on the other list. When you have say like 500 extensions and an extension you want to move is at the bottom you can't scroll up. So perhaps this can also be improved? See attached screenshot for description.