Rebex Mail implements SMTP, IMAP and POP3 protocols. None of them include the ability to change the user's password.
Each mail server has it's own way of doing so. Some of them has only web interface for this task, some can be updated via COM interface, some might expose the webservice or REST API (to name a few possibilities).
If you update the question and add which email server you are using it might be possible to come with solution. However, the universal solution does not exist.