If you have a Windows Server 2016 running, want/need to do an in-place upgrade to Server 2019, and you're not some company, you have a problem: Unless you have a MSDN subscription, you can't really get an installation ISO image. You will only get an evaluation image. Even if the language is the right one, you can't keep your data with this image. There appears to be a working way to do this, though:
Screenshotted because Internet, you never know. With this I was able to do an in-place upgrade of my mail rules server (strange construct, but it works for me). To activate it with my purchased OEM key I had to do 2 steps in a powershell admin shell:
DISM.exe /Online /Get-TargetEditions
DISM /online /Set-Edition:ServerStandard /ProductKey:ENTER-YOUR-SERIAL-KEY-HERE /AcceptEula
and afterwards do the activation by entering the product key again.
You can get the retail server 2019 download and key from keyingo.com, then it makes it easier to do the in-place upgrade !