To download and install , you can use the official "Betas" feature on Steam to access legacy versions of the game. This is the most reliable method for ensuring compatibility with older mods like ProMods 2.51 or older map packs. How to Download & Install ETS2 v1.39
Download the updater to automatically bring any older version up to the latest supported legacy version. Full Install:
Since ETS2 updates automatically on Steam, you must manually select the 1.39 "Beta" branch to access it: Open Steam Library Euro Truck Simulator 2 in your list of games. Access Properties : Right-click the game title and select Properties Navigate to Betas : Click the tab on the left-hand menu. Select Version
If you don't already own the game, you will need to purchase it on the Steam Store. Once it’s in your Library, you have access to almost every version ever released. Step 2: Accessing the "Betas" Tab