That's not really doable through RoundCube. the config file is located in //config/
there are 2 files
main.inc.php
db.inc.php
You need access to the server to get these files (ftp/SSH), but that will only give you the database information. Roundcube does not handle the IMAP/POP3 user info, more than temporarily store the data (encrypted) in the database. You cannot change an emails password through it (unless you get someone elses fork of the project, and they have integrated that functionality in the program).
Wish I had a better answer...