In order to change your Unix (Heimdal) password you will need to access the "centos7" interactive pool. "centos7" is not directly accessible from the internet. You must first login to a bastion node.
If your Unix password has already expired you must first contact the IT Service Desk and request a Unix password reset: https://it.slac.stanford.edu/support
When you have a working Unix password, then:
If you are an S3DF user:
/usr/local/bin/password
and follow the prompts to reset your password.