Hi Edward Sham,
you can download the installation media for Windows Server 2019 from Windows Server 2019 | Microsoft Evaluation Center. However, you will need a valid product key to activate the operating system after installation.
Once you have obtained the installation media and product key for Windows Server 2019, you can perform a clean installation of the operating system on your server.
Please note that this will erase all data on the server, so be sure to back up any important data before proceeding with the installation.
Kind regards,
Lei