Neowin shows you how to create Windows 8.1 ISO images

The trick, which is actually quite simple and detailed below, and it allows you to create your own media to install Windows 8.1 at a later time without having to install a previous version of Windows 8 first.

Head over to http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only
Scroll down and click Install Windows 8 (This will trigger the installer assistant).
Enter your Windows 8, Windows 8 Pro or Windows 8 Upgrade product key, confirm that you will be downloading the bits for the version you paid for.

  Here is how to get the Windows 8.1 ISO and create a USB install stick