Hello Chandan Singh3,
Thank you for posting in Microsoft Community forum.
1.You can download evaluation versions of Windows Server 2019 here.
Windows Server 2019 | Microsoft Evaluation Center
2.You can find your Windows product key here.
Find your Windows product key - Microsoft Support
3.Then your server is running an evaluation version of Windows Server Standard or Datacenter edition, you can convert it to an available retail version based on the steps below.
Upgrade and conversion options for Windows Server | Microsoft Learn
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou