An ISO image must be converted into installation media stored on a DVD or a USB flash drive.
Instructions are provided on this page. Developer tools are available for download from Windows Dev Center.
Important: If you decide to go back to your previous operating system, you'll need to reinstall it from the recovery or installation media that came with your PC, which is typically DVD media.
If 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, you won’t be able to use the recovery partition on your PC to go back to your previous version of Windows.