Well... When you download the WP 3.9 files to your computer, they're zipped. If you right click on the downloaded folder and find unzip in the menu, you'll be able to find individual files once that's complete. Then you have to locate the one the author of this plugin mentioned above.
After that either using an FTP program that has access to your site files on your host's server, or perhaps your host gives you access to your site files in another way, you have to upload that specific file which is found at wp-includes/js/tinymce/wp-tinymce.js.gz to your host's server in the same folder as wp-includes..., etc. However you upload it, it will ask if you want to replace/overwrite the existing file by the same name. Select yes and you're good to go.
May need to log out of your blog and log back in or just refresh the post page so you see the visual editor.