The 1.9.1 update was the final major content patch for the PC game, adding "Big Steel Claw" and various quality-of-life improvements. To get this version safely, you should use official storefronts:
If you are looking for free content or to purchase the game safely, here are the legitimate ways to access the BATTLETECH universe: Official Free Downloads
If you're looking for free-to-play experiences in the same universe, consider:
It sounds like you're looking for information on a "free download" for version 1.9.1. It is important to note that BATTLETECH is a commercial game developed by Harebrained Schemes and published by Paradox Interactive; "free" downloads of the full game from third-party sites are often associated with pirated content, which can pose security risks to your computer.
: Frequently features sales where the base game and its DLCs (Urban Warfare, Heavy Metal, Flashpoint) are heavily discounted.
: Often includes BATTLETECH in themed strategy bundles. Safe Alternatives