There are two copies of the password-changing and app, running in the student and core regions.
The student.cs password application is found at http://www.student.cs.uwaterloo.ca/password . It has instructions for users on the login and post-login screens. User documentation pointers are found in the Math FAQ and elsewhere (canonically listed in ST#75690).
Information in this area is meant for use by CSCF staff and is not official documentation, but anybody who is interested is welcome to use it if they find it useful.