If you legally own Project Zomboid on Steam (cost: ~$19.99 USD), you can freely switch to Build 39 at any time. as a legacy version. This leads many to believe the build itself is free to the public, which is false.
For players looking into "Project Zomboid Build 39 free" versions or wanting to revisit this specific era of the Knox Event, here is a deep dive into what made this update a game-changer and why it still holds a special place in the community's heart. The Arrival of Vehicles: A New World Order project zomboid build 39 free