When you try to change the product key in Windows 8 or in Windows Server 2012, you cannot find a “Change product key” link in the System item in Control Panel.

Use this to enable the “Change product key”

1. Swipe in from the right edge of the screen, and then tap Search. Or, if you are using a mouse, point to the lower-right corner of the screen, and then click Search.
2. In the search box, type Slui.exe 0x3.
3. Tap or click the Slui.exe 0x3 icon.
4. Type your product key in the Windows Activation window, and then click Activate.