You will lose compatibility. You will wrestle with mod load orders. You will spend an evening staring at the console log.
Steam typically updates the game automatically. If you are stuck on 1.35 or need to force the 1.36 update: Open your . euro truck simulator 2 update 135 to 136 download install
If you have the standalone retail version, you must use the official updater tool: Euro Truck Simulator 2: How To Update/ Install Patches You will lose compatibility