To manage the settings for a student, login using a web browser and go to the Manage Account / Students tab. Here you can update various settings for each of your students.
You can control the current activities that students are playing with the "Change Assignment" button. Please be careful when setting a new assignment, as that will erase that student's current progress, and there is no way to undo that. Changing the birth date on this screen does not reset their assignment, so if you accidentally set their birth date incorrectly during sign-up, modifying it here will not disrupt your student's experience.
You may also add and remove students from this screen, and control the daily playtime for each student.