How has Anna changed?
Anna became stronger mentally and physically by the end of the book. She started to make her own decisions in life rather than following orders like she was accustomed to. She also liberated herself from the Puritan religion, the church, and no longer relies on "faith" but she has hope.
She also faced her fears-saving merry wick fords life
What caused her to change?
You can argue this in a no of ways, mainly due to the influence of elinor-who gave her the confidence that she needed, she educated her etc etc
Her experiences through the plague-the constant deaths, her husband, her own children and the death that surrounds her, all helped to develop her strength at the end. You can also talk about what she saw and witness through the crisis of the plague- the fact that people began to change, also influenced on her own development