Pretty straightforward.
apt-get install roundcube roundcube-core
I do have separate directories for the different domains I host so I have to upgrade their program and config code from /var/lib/roundcube/ to /var/lib/roundcube/**domain**/. I do some customization for the name of the site (ie: "**domain** Webmail instead of RoundCube Webmail") or the main.inc.php and iniset.php for the INSTALL_PATH.
That's just a quick overview. I take the contents of /var/lib/roundcube and copy it to each domain and customize the files for each domain, remove the symlink for main.inc.php and copy the file from /etc/roundcube/.
Beyond that the files between the domain and what the package installed to /var/lib/roundcube are identical. The /etc/roundcube/db.inc.php never changed beyond what was installed by default and the debian-db.php file was specified as include_one(...) which worked before, but doesn't seem to be responding now.
I've considered going back to the older version of roundcube if I can find the debian package for it, but I'd prefer to move forward....