Windows 8.1 is a free upgrade for Windows 8 owners. Unfortunately if you have Windows 8.1 disc and you want to do a fresh install, it won’t accept the Windows 8 key during the installation. To work around this, you can use the KMS Client Setup Keys during installation and enter your Windows 8 key after the installation to activate it.
As an example if you want to install a Windows 8.1 Professional using Windows 8 Professional (they both have to be the same edition), when you reboot your machine using Windows 8.1 DVD and it asked for a key during installation, you can enter the KMS Client Setup Key GCRJD-8NW9H-F2CDX-CCM8D-9D6T9 to bypass that screen. You don’t need to be connected to the Internet to do this.
Once the installation is done, connect to the Internet and open the System window.
Click on Activate Window link and this time, use your Windows 8 key to activate it. This is the best way to do an update to windows 8.1 without going through two installations. That’s all there is to it.
What if you lost or misplaced your Windows 8 product key?
Another Method
If you want to upgrade an older OS (XP or Windows 7) and you have your Windows 8 key, you can download the installation file from Upgrade Windows with only a product key page. Make sure you backup everything you want to keep and read the instructions carefully before deleting anything.
This method only works if you already have a running OS and it won’t work to upgrade a 32-bit version to a 64-bit.
Further Reading
KMS Client Setup Keys
Upgrade Windows with only a product key
