If you specifically need version 1.26 (often for mod compatibility), you can right-click the game in your Steam Library, select Properties > Betas
If you want a lightweight version of the game that still captures the "zen" of the open road, V.1.26.2.4 is a great choice. It’s a solid, reliable build that highlights how far the simulator has come while remaining perfectly playable today.
Euro Truck Simulator 2: 1.58 Update Release - SCS Software's blog