How does the society change Montag in Fahrenheit 451?
Society does not directly "change" Montag during the story; in fact, the changes he undergoes come from his personal introspection and from his conversations with Clarisse. In the most literal sense, Montag is entirely a product of the society at the beginning; he is unable to think past the indoctrination given to all people growing up in this society. However, it could be said that his increased awareness of the problems in society cause some of his changes; as he sees past the propaganda and into his own rational thought, things that he never considered push him farther away from the society.
"People don't talk about anything."
"Oh, they must!"
"No, not anything. They name a lot of cars or clothes or swimming-pools mostly and say how swell! But they all say the same things and nobody says anything different from anyone else. And most of the time in the cafes they have the jokeboxes on and the same jokes most of the time, or the musical wall lit and all the coloured patterns running up and down, but it's only colour and all abstract."
(Bradbury, Fahrenheit 451, hrsbstaff.ednet.ns.ca)
These things, of which Montag is unconsciously aware, had seemed normal to him until he met Clarisse. Now, he cannot help but be hyper-aware of all the strange things and irrational behaviors that drive the society. He is increasingly irritated with television in particular, as the shows have no story or content, just disconnected emotions. In that way, Montag's self-awareness allow the negatives of society to change him in a positive way.