How have Biff, Happy, and Linda changed by the end of Act II in Death of a Salesman?

Throughout his life, Biff blames Willy for his own shortcomings. We find that Willy spoiled Biff, leading him to believe that he could behave any way he wanted. When all the success fails to materialize, Biff is tortured by his disillusionment with Willy: by his failure to live up to his own standards, by his failure to achieve the greatness that Willy dreamed he would, by his desire to get back at his father for what he believes has been done to him. But all of this is contrasted with his great love for Willy, which creates in him tremendous confusion and emotional turmoil. However, in the Requiem scene at the play's end, Biff illustrates that he has truly come to an understanding of his father's failure to achieve success, observing that Willy "never knew who he was" and that he "had the wrong dreams." So Biff has changed. He's learned to accept his father, and take responsibility for his own life. He also shows respect for Willy for the first time in the play.

Happy does change, but it is not necessarily for the better. As the younger of the two, he has grown up in Biff's shadow, and consequently has a bitterness that Biff does not. He appears more content than Biff, but his own disillusionment is revealed in his self-created job failures, as well as the numerous affairs he carries on. At the play's end, he is somehow converted by Willy's dreams; he pledges to take up his father's cause and succeed where his father had failed. While after Willy's death Biff recognizes his father's failings, Happy wildly proclaims:

"I'm gonna show you and everybody else that Willy Loman did not die in vain. He had a good dream. It's the only dream you can have—to come out number-one man. He fought it out here, and this is where I'm gonna win it for him."

Throughout the play, Linda plays the dutiful, if deluded wife. She seems to believe all of Willy's wild ideas, and willingly supports him despite his overt cruelty to her. When no one shows up for his funeral, she is visibly upset and confused. This demonstrates that she has not fully recognized – or refuses to recognize – Willy’s “phoniness.” Nevertheless, her disappointment also expresses her conviction that Willy deserves respect because he tried bravely to succeed even though he was not “a great man” but only a “human being,” “a little boat in search of a harbor.” Her words, “We’re free,”  acknowledge the irony that Willy has killed himself just when their lives might have begun to improve, due to the end of the house payments. In another sense, Linda and the rest of the family are free from Willy, someone who often made their lives difficult. While they never wanted Willy to kill himself, his absence may let them view life in a new, clearer way. Thus Linda becomes the one character that truly understands Willy, and what he has attempted to do throughout his life.