Go to "My story" and select "See all" under one of the categories. Here you see all the answers you gave for that category. If you press the small pen next to a question, you will be able to change your answer.
When you have changed the answer to a question, some of the follow-up questions will also change, and some of the answers you previously gave will disappear. For example: if you change the answer to whether you go to school or not, the questions you answered about homework and schoolwork might be irrelevant. This means that you will answer similar questions again that you have already answered. But it's not a problem, and you don't have to do anything special other than answering questions and telling more about yourself.