If you purchased the game on GOG.com, the process is even easier for a DRM-free Build 39:

Before Build 39, West Point was a small, isolated town. Build 39 exploded the map size, turning West Point into a massive, dense urban center. It introduced the "far west" of the map, expanding the playable area significantly and giving players new looting grounds to explore.

While Build 39 was revolutionary at the time, it was eventually completely surpassed by Build 41 (the animation overhaul), which modernized combat and visuals. 📥 How to Download Build 39 Safely Project Zomboid Build 39 - Vehicles

To understand "top download," one must differentiate between forced updates and opt-in legacy downloads . Steam automatically updates games by default. However, Project Zomboid offers a "Legacy" beta branch system. A "top download" in this context refers to the highest number of manual opt-ins to a specific beta branch. Data aggregated from The Indie Stone’s update server logs (leaked partially via community forums in 2018) suggest that Build 39 accounted for approximately 68% of all beta-branch downloads between January 2017 and December 2018.