Going through this myself now. Officially you have to buy the "Pro Pack" from Microsoft which is 50 (yup, I'm still British!). Luckily I had a 50 voucher from when they messed up my Surface order so I just used that, however if you don't want to pay that amount I would suggest the following:
- Find a Windows 7 machine (any kind will do)
- Go through the upgrade process which allows you to buy the pro upgrade for 24 (or 14 if you have a Windows 7 key you can use to get the discount)
- Don't upgrade that machine, instead choose to download the iso image
- Using the Windows 7 USB iso tool, create a bootable USB drive
- Now install Windows 8 pro using that USB drive on your laptop. The license key will work but it will need to be a clean install
Unfortunately it's not the simplest of processes but it is the cheapest. Alternatively if you want the quick easy route you can just search "upgrade windows" in your Windows 8 start screen and follow the on screen instructions to download/install the pro pack. It will just cost you that full 50.