Rollback 5,1,1

Ok I jus got an unrooted phone to rplace my s5 and would like to get it rooted but what I see is that I need to roll back. Where do I find those files at, and the do i just flash in odin then root with cf autoroot? If someone would steer me in the direstion of the files I need that would be freat. By the way mine is stock s5 running 5.1.1 base g900tuvu1f0g6. thanks for any help