Download Windows 8.1 .iso

To clean install Windows 8.1 .iso using a Windows 8.0 retail product key follow the instructions here:

Download Windows 8.1..iso

And:

Windows Reinstallation Guide/A Clean Install of Windows 8.1

Unfortunately there is no current solution to download a Windows 8.1 .iso for a preinstalled (OEM) version of Windows 8.0 or 8.1

You may also want to have a look at customising Windows 8.1:

http://philipyip.wordpress.com/customising-windows-8-1/