To , the most reliable method is through the Steam store or the official Euro Truck Simulator 2 website. If you are looking to revert to this specific version from a newer one, you can do so via the Betas tab in your Steam library properties. Key Features of the 1.39 Update
Released on November 3, 2020, version 1.39 introduced several major overhauls to the base game: ETS2 1.39 Update Release - SCS Software's blog
