To download Windows Server 2012 R2 Evaluation iso file you click these links below:

This Windows Server 2012 R2 iso file includes Windows Server 2012 R2 Standard Evaluation and Windows Server 2012 R2 Datacenter Evaluation

Step 1: You open cmd (run as administrator), then you copy and paste this code:

dism /online /get-targeteditions

You will see Windows Server 2012 R2 editions you can upgrade

Step 2: You copy and paste this code in cmd

If you want to upgrade Windows Server 2012 R2 Evaluation to Standard you copy and paste this code:

dism /online /set-edition:serverstandard /productkey:D2N9P-3P6X9-2R39C-7RTCD-MDVJX /accepteula

If you want to upgrade Windows Server 2012 R2 Evaluation to Datacenter you copy and paste this code:

dism /online /set-edition:serverdatacenter /productkey:W3GGN-FT8W3-Y4M27-J84CP-Q3VJ9 /accepteula

Done.