If your password is not working you can check with an account administrator to make sure your credentials have not been changed. If your account is connected to LDAP, you will need to make sure you are using your network username/password combination.
Teachers may need to contact their account administrator in order to reset their password.
You are also able to use the "Forgot Your Password?" link on the sign in screen to reset your password if your email address is associated with your account.
The password change is performed on the My Profile -> Change Password section of My Settings.