Annoyingly there’s an icon to tell you to “Get Windows 10” in the system tray that won’t go away even if you complete it’s reservation process. Fortunately, there’s a way to get rid of it.
To get rid of the “Get Windows 10” icon from the system tray, simply uninstall and block this Windows update: Update for Microsoft Windows KB3035583.
Unfortunately, not having this update may mean that your computer won’t automatically upgrade to Windows 10 at the end of July when it comes out.
My advice: Put up with it for the remaining month or so that you have to wait, it’s not a big deal. If you absolutely have to get rid of the icon from the system tray, maybe because you’re managing a bunch of Windows 8 computers in a corporate network and it’s generating service desk calls, your best chance is to look into how to hide specific system tray icons via GPO and do it that way.