Throughout the story, baba worries because amir never stands up for himself. when does this change?

From the above given sentence we can find that it is about "The Kite Runner".

This changes will happen when Amir is an adult and he returns to his native country to find out that his friend has died, and he vows to take care of his nephew.

He, after living in America, finally matures into a man and now he is ready to fight for himself. he will face the problems himself and solve them in his own way.