Hi,
since this project is mostly abandoned you will need to do it yourself. You can replace the roundcube entries in /your/installfolder/mailinabox/setup/webmail.sh like described here: roundcube-1.6.15 by jcm-shove-it · Pull Request #2565 · mail-in-a-box/mailinabox · GitHub
Then re-run the command to update your MIAB installation. For my system I follow this Mail-in-a-Box Maintenance Guide
This re-runs the setup procedure and should update Roundcube automatically to the new version.
This worked fine for me
Good luck