To begin, you should note that from RoundCube v1.0 and onwards, the configuration settings are included in one file only, as opposed to previous versions where they were split between two files.
First, locate the following file and make a copy named config.inc.php in the same directory. Use the -p option to preserve mode, ownership, and original timestamp:
# cp -p /etc/roundcubemail/defaults.inc.php /etc/roundcubemail/config.inc.php