Not a lot to it, just a link
Upgrade to Windows 8 with only a product key
If you bought Windows 8 and you have a Windows 8 product key, you can download and install it from this page.
Before you begin:
Make sure you’re on the PC you want to upgrade. This PC must be running Windows 7, Windows Vista, Windows XPwith Service Pack 3 (SP3), Windows 8 Release Preview, Windows 8 Consumer Preview, or Windows Developer Preview. (Windows 8 Setup checks that your PC meets all system requirements before it installs Windows 8.)
Have your product key handy. You can find it in your online order info, in the confirmation email for your purchase, or on the packaging. The product key will look something like this:
XXXXX- XXXXX- XXXXX- XXXXX- XXXXX
Make a backup of everything you want to keep. Save it somewhere you can get to it later, like an external drive or a DVD.
Don't format your hard drive before installing Windows 8. Your current version of Windows must be running when you begin installing Windows 8.
When you’re ready, click Install Windows 8 to start upgrading this PC.Install Windows 8