Upgrade Windows 7 Bootcamp partition to Windows 8.1 with installation disk

Hello, I have a Late 2011 15 inch MacBook Pro with a bootcamp partition containing Windows 7. My school has a website where I can get free copies of over 200 Microsoft products, Windows 8.1 being one of them. I have downloaded the Windows 8.1 .iso file, and I would like to upgrade my Windows 7 partition to Windows 8.1, keeping all of my important school files. Is there a way to do this with the DVD containing a Windows 8.1 .iso with a valid activation key? Any help is greatly appreciated!

 

I want to stick with Windows 7 too, but I need to use Win 8 for a certain program for a school project..