Windows Server 2008 R2 includes a command-line tool called the Deployment Image Servicing and Management tool, or DISM. It’s functionally equivalent to the Windows Anytime Upgrade feature in client versions of Windows. You can run it from an elevated command prompt.

To determine the currently installed edition:

DISM /online /Get-CurrentEdition 

To determine the editions you can upgrade to:

DISM /online /Get-TargetEditions 

To actually perform an upgrade:

DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX