You can change your TEACHING password by logging into linux.student.cs.uwaterloo.ca via ssh, running the command passwd, hitting return, and supplying your current password at the prompt, followed by your new password (twice).
If you cannot access your account, you can use this application to self-service reset your password. It will ask you to log in with your WatIAM/UWDir password and then allow you to change your TEACHING password or reset your Linux shell to /bin/bash.
Setting your GENERAL Password and Shell
Your TEACHING account is different from your GENERAL account (details here). If you want to reset your GENERAL password or shell, you can do so at cs.uwaterloo.ca/password.