The final versions of Windows 8.1 and Windows RT8.1 are available online and in windows store for download. Windows 8.1 preview is no longer available for download. You must install the final version of Windows 8.1 or Windows RT 8.1 before January 2014 when the license the preview will expire. If you’ve been using the preview, you should update to the final edition as soon as possible to avoid an interruption in using your PC.
For up-gradtion help to windows 8.1 from Windows vista or Xp , refer here
For buying Microsoft Windows 8.1, Visit Microsoft Online Store