Open a command prompt and enter the command:
DISM /online /Get-CurrentEdition
to find the current server edition.
After that enter the command:
DISM /online /Get-TargetEditions
to find the editions that your system can be upgraded to.
Finally enter the command
DISM /online /Set-Edition:ServerEnterprise /ProductKey:XXXX-XXXX-XXXX-XXXX-XXXX
to upgrade to enterprise version. Of course you must use your real enterprise product key.
If you have a KMS to activate your windows server then you can use the known KMS client key, so the command becomes:
DISM /online /Set-Edition:ServerEnterprise /ProductKey:489J6-VHDMP-X63PK-3K798-CPX3Y
For more information read the following article.
There is a possibility that after this upgrade procedure the system does not allow you to use memory above 4 GB and the allow remote desktop check box is disabled. In this case use the following procedure to fix this problem:
Open a command prompt
Type slmgr.vbs -ckms (this clears any KMS entry you have)
Type slmgr.vbs -upk (this removes any product key installed)
Type slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x’s is the new enterprise product key you will use)
Type slmgr.vbs -ato (this activates the server)
After this you must restart the server, so that you can utilize memory above 4 GB and to enable remote desktop check box in system properties.