Scroll through the available historical versions. Steam often archives major milestones (such as 1.0.5, 1.3.1, or 1.7.3).
You will not find this on Steam unless you are a time traveler. Steam auto-updates to the latest 1.12.x. To get this specific build (0.25.0.642), you have two options: Kerbal Space Program -KSP- 0.25.0.642 download for computer
Playing KSP version 0.25.0.642 is a fantastic way for both new and experienced players to appreciate the game's development journey. It's a snapshot of a time when Career Mode was being truly fleshed out, stock spaceplanes were getting a major overhaul, and the foundation for many of the game's future features was being laid. Scroll through the available historical versions