In Freak the Mighty, what are two differences between the book and the movie in which the director changes something in the movie version? I would like to talk about the following differences: 1 - The quote Loretta said about Freak being a birth defect in the book; Freak says it about himself in the movie. 2 - The movie shows a bigger picture of the negative effect Killer Kane had on his family; I need to explain how.

Since you have already outlined two differences between the book version of Freak the Mighty and the movie adaptation, let's focus on explaining how and/or why those differences manifest. 
First, in Chapter Eleven of the book, Freak and Max go to the house of Loretta Lee in order to return her purse to her (which they had discovered in a storm drain in the previous chapter). Once there, Loretta behaves with extreme nastiness toward the boys, pointing out that Max is the son of Kenny "Killer" Kane and must be "some kind of retard," while Freak is the son of "stuck-up Gwen" and a "dwarf." She points out with great cruelty that Freak's father had abandoned him, stating:

Your old man was a magician.... He must be a magician, because as soon as he hear the magic words "birth defect," he disappeared. 

In the film version, it is not Loretta who makes this statement, but rather Freak himself. After Loretta reminds Max about his father, she insults him, recalling that he was just a "poor little mute boy, all alone up there on the witness stand." Max takes off running and tells Freak to stay away from him, claiming that he's been "nothing but trouble" and that his "life was fine before you and your stupid quests." Freak then tells Max:

My father was a magician. At least that's what my mom says. He heard the words "birth defect" and he disappeared. But I don't really care, you know, 'cause I don't even know who my father is. I just know that's not who I am. And that's not who you are, either. 

The director of the film, Peter Chelsom, and writer, Charles Leavitt, clearly decided to make this change to give Freak more agency. Rather than being hurt by this truth, he is owning it and using it to empower Max to realize that he is not his father; he is not destined to be a killer. It becomes a moment of liberty rather than a moment of injury. 
Second, as you pointed out, the film shows the larger consequences of Killer Kane's actions in that we get a more thorough perspective from Max and his grandparents.
In the film, we clearly see the bullying that Max faces as a result of his father's actions and the negative impact this has on the boy's psyche. He's called at various times "a freak of nature" and a boy with "no friends." He's blamed for creating trouble that he did not create and given detention. He is also subjected to repetition of the terrible song that the other children have made up: "Killer Kane, Killer Kane, had a son who's got no brain!"
This treatment results in Max retreating into his head to the "cool and dim" place where he can "float like a cloud." He is clearly withdrawn and unable to connect with others as a result of his father's behavior. He lives in constant fear that he might turn out to be like his dad, stating, "I look in the mirror and I see him. I hear my voice and I hear his!" This insight is aided by the voiceover narration provided by Max throughout the film.
We also see how this has devastated Gram and Grim, who have to live with the knowledge that their daughter's husband murdered her. Grim keeps a gun in the house in order to shoot Kane if he ever comes back, which Gram detests. She states emotionally:

Our Annie's gone. All we have left of her smile... all we have left of her heart is in that boy down there. I'm not gonna have hatred coming into this house and poisoning him. 

The revelation that Kane will be released shortly on parole creates tension in the family in that they don't know what to believe; lawyers claim that Kane will be put back in prison if he comes within five miles of Max or the house, and yet it is lawyers who have enabled Kane to be released in the first place.
All of this conflict naturally comes to a head when Kane kidnaps Max and takes him to Loretta's house, where he tries to strangle her. Seeing this triggers Max's memory of his mother's murder, and he is finally able to own the situation and free himself of its emotional weight, stating:

I wasn't dreaming. I remember. I saw you! I saw you kill my mom! And you're never gonna stop me from saying it again!