Changing Product Key on Window8 and Window Server 2012

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>

Reference: http://support.microsoft.com/kb/2750773