Hi Clement Mezerette,
to perform an in-place upgrade while keeping your data and files, you will need to have a valid license key for Windows Server 2019 Standard edition.
You can purchase a license key from Microsoft or a Microsoft reseller, and then use that key to activate the trial version of Windows Server 2019. Once activated, you should be able to perform an in-place upgrade while keeping your data and files.
Alternatively, you can perform a clean installation of Windows Server 2019 and then migrate your data and files from the old server to the new server. This may take more time and effort, but it will ensure that you have a clean and stable installation of Windows Server 2019.
Regards,
Lei