To update your interests, click your name in the top-right corner of the screen and then click "Interests." You'll then be able to scroll through and select or deselect any of the boxes, or enter the names of your friends and pets at the bottom.
Please note that updating your interests will not affect quizzes and assignments that have already been created by your teacher before you updated your interests. If you try doing some questions in "Practice" mode, you'll see your new interests in action!
Teachers: You can update your interests by navigating to the "Settings" page and then clicking the blue "Update Interests" link at the top of the form.