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

DBGBW-NPF86-BJVTX-K3WKJ-MTB6V