Hi ChrisD_2501
You can download the non-evaluation ISO for Windows Server 2016 from the Microsoft Volume Licensing Service Center (VLSC) if you have a valid license key.
You can also download it from the Microsoft Evaluation Center and convert it to a non-evaluation version using the DISM tool. Please refer to:
Windows Server 2016 | Eval Center (microsoft.com)
To perform an in-place upgrade, you can mount the ISO and run the setup.exe file from within Windows Server 2012 R2. Make sure to back up your data before performing the upgrade.
Best Regards,
Hania Lian
============================================
If the Answer is helpful, please click "Accept Answer" and upvote it.