First, keep in mind that parents and students may share one set of credentials. If you change your password, be sure to communicate this so someone else doesn't get locked out. The following instructions only work if you can already log in. If you can't, contact your school for assistance.
Once you're logged in, click the icon in the upper right, then choose "Change Password". The next screen will provide instructions and requirements for your new password. When you've entered and confirmed an acceptable password, click "Save".