Assuming you already own ETS2 on Steam (or a retail version linked to Steam), follow these steps to get version 1.42.
All major updates for Euro Truck Simulator 2 are free of charge. If you already own the base game on a legitimate platform (such as Steam), you do not need to pay for the update. The game is supported by paid DLCs (map expansions, truck packs), but core engine updates and map reworks are provided for free to all players.
You can download the game via the Steam Store or the Epic Games Store. The game utilizes a "Freemium" model where the base game is sometimes offered for free during promotional periods, or you can purchase it to support the developers. Once installed, you will have access to all updates.