I have a Windows 8.1 Professional Key, and I am running Windows 8.1 which is already activated. Can, and how, do I upgrade to professional? It doesn't work in the Windows Anytime Upgrade.
Here are the official instructions from Microsoft :
In short, you'll have to go into the Windows Store, and find Windows 8.1.
During the installation, you'll have to give the CD Key you bought.
Windows Anytime upgrade is only meant to change between different editions of the same OS (8 an 8.1 are seen as two distinct OS).