Windows Server 2012 R2 Evaluation to Standart Edition
Open cmd with admin privileges
DISM /online /Get-CurrentEdition
After knowing your edition then you can see your upgrade options with
DISM /online /Get-TargetEditions
Finally change edition id to your upgrade edition and enter your product key.
DISM /online /Set-Edition: /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
Restart your server.
Licence key for standart edition