Finally Windows 8.1 Preview is available for download. To install the preview you need to install an update.

Visit the Windows 8.1 Preview download site to download and install the patch. Click on Get Update button.

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The patch installation requires a restart.  Once you install the patch and restart the PC, go Store and it will prompt you to download Windows 8.1 Preview

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The whole download and installation process may take an hour or more and would need a couple of system restarts.

 

Install Windows 8.1 Preview from an ISO file

If you decide that you want to install Windows 8.1 Preview using the ISO, you won't be able to uninstall it. If you decide to go back to your previous operating system, you'll need to reinstall it using the recovery or installation media that came with your PC, which is typically DVD media. If you're running Windows 8 and you don't have recovery media, you might be able to create a USB recovery drive. If you're running Windows 7, Windows Vista, or Windows XP and you don’t have recovery media, you might be able to create it from a recovery partition on your PC using software provided by your PC manufacturer. Check the support section of your PC manufacturer’s website for more information. After you install Windows 8.1 Preview, you won’t be able to use the recovery partition on your PC to go back to your previous version of Windows