In Windows 8 you don’t have the option to change the product key in the computer properties window.

So here is how to change the product key in order to activate it.

To change the product key without first activating Windows, use one of the following methods:

Method 1

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 3.
   

3. Tap or click the Slui 3 icon.
4. Type your product key in the Windows Activation window, and then click Activate.
   

Method 2

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. Type Command Prompt in the Search box.
   

3. Right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
   

4. Run the following command at the elevated command prompt:
   slmgr.vbs /ipk <Your product key>