Microsoft has finally released the ISO images of Windows 8.1 Preview

How to install Windows 8.1 Preview from an ISO file

To install Windows 8.1 Preview from an ISO file, you must first convert the ISO file into installation media stored on a DVD or a USB flash drive.

If you're using Windows 8, follow these steps to install Windows 8.1 Preview from an ISO file:

Download the ISO (.iso) file.

Double-tap or double-click the ISO file.

Double-tap or double-click setup.exe and follow the steps.

If you're using Windows 7, the easiest way to convert an ISO file to a DVD is to use Windows Disc Image Burner. On a PC running Windows XP or Windows Vista, a third-party program is required to convert an ISO file into installable media—and DVD burning software often includes this capability.
Important

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.
Notes before you download

Windows 8.1 Preview is prerelease software that may be substantially modified before it’s commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here. Some product features and functionality may require additional hardware or software.

  Windows 8.1 Preview ISO Images Available