(QuickPacket sells Windows licenses at a great price, order a server with us and get 15% off your first invoice with promo code PACKETS)
To change your Windows Server 2012 license key open power shell in privileged mode and enter the following commands:
1. To remove your existing (default) installation key:  slmgr -upk
2. To add a new key slmgr -ipk XXXX-XXXX-XXXX-XXXX
If you’ve entered a valid key, your Windows Server will be activated automatically in a few seconds, no additional steps are required. You will no longer see a warning message on your desktop and in Control Panel / Actions.