You may for some reason need to change your Windows product license key – maybe you want to upgrade your copy of Windows to a higher version or maybe you want to change the default setup product key to a Multiple Activation Key. Follow these steps if you want to change your Windows product key.
Change your product key in Windows 7
If you wish to change your product key in Windows 7, click the Start button > Right-click Computer, and then select Properties.
Scroll down to the bottom of the window. Under Windows activation, click on Change product key.
Follow the instructions to change your product key and activate your copy of Windows 7. More on this here.
Change your product key in Windows 8
Follow these steps to change your product key in Windows 8. Open the System properties applet in the Control Panel of Windows 8; Control Panel\System and Security\System. You may also open the Win+X menu and select System. Click on Get more features with a new edition of Windows.
The following window will open. Click on I already have a product key.
Enter the product key and click on Next. Windows will now validate and then accept the key.
You can also change Windows product key by typing the following in an elevated command prompt and hitting Enter:
slmgr.vbs -ipk <Your new Windows product key>
Microsoft has simplified this process in Windows 8 and all you need to do is, open Control Panel and navigate to System and Security > Action Center > Windows Activation. If you have not entered your Product License Key and/or not activated your copy of Window 8, you will see a note to this effect in the Action Center. Click on the link to enter your License Key and Activate it. Make sure you are connected to the Internet.
You can also type slmgr.vbs -ato in the elevated CMD to activate your copy of Windows.
Go here if you find that the change product key link not available.