I know it is recommended to perform a clean install for each build but here is how i performed an in-place upgrade from 18317 to 18334
1- Download the Windows Server Insider .iso
2- Extract the content or mount the .iso using PS command Mount-Diskimage
3- Shutdown any VM(s) located on your server (that prevents the upgrade if a VM is On)
4- Check your current build: Get-CimInstance Win32_OperatingSystem (18317 for my test)
5- From the extracted .iso folder or .iso mount point run below command line:
.\setup.exe /auto upgrade /quiet /migratedrivers all /dynamicupdate disable /showoobe none /imageindex 2
NOTE: set imageindex to 1 for Standard edition or 2 for Datacenter edition
6- You can remotely monitor from your workstation the first part of the upgrade opening setupact.log from C:\$Windows.~BT\Sources\Panther - I suggest to use CMTRACE.exe to do this
7- After a couple of reboots the upgrade should complete successfully. Check again the build version using "Get-CimInstance Win32_OperatingSystem" cmdlet (18334 for my test)
Build-to-build upgrade of Windows Server is now supported. The notice that recommended a clean install has been removed from the product. You may now upgrade from preview build to preview build.
I'm not sure about official documentation other than the links I provided, but you should be able to just run Setup and keep installed apps and files.