You can download the update through the platform where you originally purchased the game:

KSP 1.12 is considered the "Definitive Edition" of the original game. It offers the most stable platform for mods and is the version supported by almost the entire modding community today. Enjoy your flights

Are you planning to install specific like Realism Overhaul or Galactic Neighborhood on version 1.12?

As the sun set over the KSC, Jeb looked up at the stars. This was the final major chapter for Kerbal Space Program 1

Before you search for "Ksp 1.12 Download," you need to understand why version 1.12 is the holy grail of KSP.