Change Product key and Activate Window 8 Enterpris RTM

So yesterday I received an mail from an colleague of mine asking if i was able to activate my newly installed  Windows 8 Enterprise RTM machine.
Since it wasnt activated I got this watermark in the right lower corner

Normally you have an option that allows you to change the Product Key. Like the screenshot below

This option however was not to be found on my machine

So what now?

Well if its not able to do in GUI, it will be by using Command Promt.

Start and cmd by right-clicking in the down-left corner and choose Command Promt (Admin)

Type the this command:

[important]SLMGR.VBS /IPK XXXXX-XXXXX-XXXXX-XXXXX-XXXXX[/important]

And than you will receive an message telling you that it has been installed.
Finish by activating your installation with the newly changed cdkey:

[important]SLMGR.VBS /ATO[/important]

You will notice now that it has been activated the watermark has disappeared.

Keep on enjoying Win8!