If you’re ready to bring back the Start button, you can head over to Microsoft's website to download the Windows 8.1 Preview right now. The Preview release is open to all users, though any machine that isn't running Windows 7 or later will have to perform a fresh install to get it working. For those already on Windows 8, Microsoft is offering the 8.1 Preview directly through the Windows Store. To access it, users have to first visit Microsoft's website to initiate a small update to Windows itself. Afterward, their device will restart and direct them to a download of the 8.1 Preview update.
Update: Microsoft has opened the Windows 8.1 Preview up to all users. This article has been updated to reflect the additional availability.