For those that are just getting started with Windows Server 2012 you may have noticed that you don’t get the option to change the server’s product key on the activation page. This can be a problem if you need to switch to MAK or KMS keys and reactive the server. No worries, the solution is pretty simple.  You can use one of the following 2 options:

Option #1 –

1. Open a Command promtp and run the following :

Option #2 – 

1.  On Windows server 2012  -( with your mouse point to the lower-right  hand corner of the screen)  click Search.

2.  Type Slui.exe 0x3

3.  When the activation screen pops up type in your new Product Key and Activate.