If you have the original retail or standalone version, you can find manual patches:
If you own a physical DVD copy or a non-Steam digital license (from the official SCS Shop), you can download the official patch installer from the SCS Software archive. Do not use third-party mirrors like Softonic or Uptodown, as these often bundle adware. Download Euro Truck Simulator 2 Version 1.5.2
Official ETS2 Website : Provides links for various legacy updates. 3. System Requirements (v1.5.2) If you have the original retail or standalone