Use the dsim tool for completing this update. Start a command console and run the following command
Dism /online /Set-Edition:ServerEnterprise /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
where XXXXX-XXXXX-XXXXX-XXXXX-XXXXX is your product. Key. If you get an error about the key not being valid you can use a microsoft provided one – 489J6-VHDMP-X63PK-3K798-CPX3Y followed by activating using your key i.e.
Dism [...]