You cannot download version 1.35 from the Steam store page directly (only the latest version is shown). However, Valve allows you to roll back to any previous version via the "Betas" tab. Here is how to do it:
: In most cases, Steam handles updates automatically. However, if you're playing a version that's significantly outdated, you might need to manually update through Steam. Download Euro Truck Simulator 2 Version 1.35