Click the Log in link on the any page (or go to https://math.sierracollege.edu/user/login) and enter your username (probably the same as your Sierra username) and your password (your password is different from your main Sierra password, unless you change it to be the same).
Click your username in the User Profile menu (1). Click the Edit profile link (2). Enter your current password (3). Enter and re-enter your new password (4). You can also update your email address; this will only change the email address that the website sends notifications to; your school email address will be the only publicly visible email address on the site.