Normally, to change the product key in Windows 8, you open Control Panel, select System, and then click on the Change product key link.

If when you try to change the Windows product license key, you find that the Change product key link is not available or visible, you may want to check if your Windows is activated first, because this usually happens if Windows has not been activated yet.

You can activate Windows using the SLUI.EXE 3 command. You can get more information here on how to activate any version of Windows.

Or you can run the following command at an elevated command prompt says KB2750773:

slmgr.vbs /ipk <Your Windows product key>

It should work!