Since ETS2 is almost exclusively distributed via Steam today, the game automatically updates to the latest version. To play the "classic" version 1.40.4.8, you would need to . Steam makes this possible through its "Betas" feature:

The safest way to enjoy Euro Truck Simulator 2 and its updates is through authorized distribution channels.

Whether you're running the legacy version or the modern Steam edition, here is how to handle your product key: Steam Activation

While you may want to stay on version 14048 for a specific mod pack, a legitimate key allows you to safely move to future versions through Steam.

If you purchased a physical copy or bought directly from the SCS Software website, a key is sent via email or found inside the box.

Old retail boxed copies of Euro Truck Simulator 2 Gold Edition often shipped on disc with version 1.48 as the base install before online patching.