Hello, If you purchased a Windows 8.1 Pro Key from me this is how you install it.
Step 1: Downloading Windows 8.1 Pro
Go to http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only and click on "Windows 8.1" (Don't worry, it will be pro)
Step 2: Opening
It should start downloading, then open up the .exe file.
Step 3: Entering the Activation Key
Enter the Activation key I have provided to you through eBay messages. After that click continue and follow the on screen instructions. That's it!