Assuming your using Windows, You can download Unity and there should be a button like 'open project' or 'open folder' then click the folder with the code in it, then Digital Logic Sim is open in Unity. If you have Notepad or Visual Studio (Code) then you can edit the code in there.
For a more in-depth explanation someone else in the community might know, I've personally never experimented with the code of Digital Logic Sim that much, just opened the code in Unity.