Please give at least one specific example of how Peeta changes through The Hunger Games.
You might like to think about how Peeta is presented in Chapter Nineteen of this excellent novel, when Katniss finds him and takes him to the cave and sees the full extent of his wound. When Peeta saved Katniss from being killed by Cato, Cato wounded Peeta badly in his leg, and it is only in this chapter that Katniss discovers just how badly Peeta is wounded. Given the nature of their environment and the lack of medical supplies, the chances for Peeta are pretty slim. Peeta is obviously changed by the reality of his situation. He recognises the way that he is unlikely to survive the experience. This of course develops and matures him. Note how he tries to raise the subject with Katniss, only to be interrupted every time:
"Look, if I don't make it back--" he begins.
We can see the acceptance that Peeta has of his death again in Chapter Twenty Five, when he insists that Katniss kills him to win the Hunger Games, and then rips of his bandage to his wound so that he will die. Peeta has so clearly changed from the young and innocent young man who worked in the bakery and gave bread to Katniss so long ago.