How to download the version of UE4.23 source code. I can saw the source code zip files in release but I don’t know where is the required 1 of 2 and 2of 2 zip files,
You can get the source code through their github repo, though I strongly suggest that you do not do that, unless you know what you are doing of course 
Documentation: Downloading Unreal Engine Source Code | Unreal Engine Documentation
Not sure what level you’re at, but this 11 minute video was my introduction to building from source and it explains how to download different versions of Unreal to build from as well.
