Well it also gives you that option when you boot, "Windows 10" or "Prior Installation". This assumes you did not delete your "windows.old" folder, in which case you are SOL.
This is why i asked for your method. This is what i tried, and after a lengthy reboot, it just took me straight back to the "continue to tech preview" blue screen. I installed over 8.1, not a clean install, windows.old still there taking up a big huge massive chunk of storage.