To enable the email account password change option from roundcube, you can add the following in file:
/usr/local/apache/htdocs/roundcube/config/main.inc.php

Don’t forget to replace your postfix user “PASSWORD”

$rcmail_config['plugins'] = array('password');
$rcmail_config['password_driver'] = 'sql';
$rcmail_config['password_db_dsn'] = 'mysql://postfix:PASSWORD@localhost/postfix';
$rcmail_config['password_query'] = "UPDATE mailbox SET password=CONCAT('{PLAIN-MD5}', MD5(%p)),modified=NOW() WHERE username=%u LIMIT 1";

You can find your postfix password here:

grep pass /etc/postfix/mysql-virtual_mailbox_maps.cf