1.To determine the installed edition, run:
DISM /online /Get-CurrentEdition
2.To check the possible target editions, run:
DISM /online /Get-TargetEditions
3. Finally, to initiate an upgrade, run: (you must input the MS default KMS key for datacenter)
DISM /online /Set-Edition:ServerDatacenter /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
4. reboot the server, and input your key and active the server.
note: MS default KMS key for datacenter
http://technet.microsoft.com/en-us/library/ff793421.aspx