To update the centrally installed RoundCube on your control panel server, you will need to know the unix user of your RoundCube website. Unless you have deleted and re-created it since the initial installation of Enhance, this will be webmail_1. If not, you can find it in /etc/passwd, it will have a home dir matching the website ID of the RoundCube website.
su - webmail_1
wget https://github.com/roundcube/roundcubemail/releases/download/1.6.4/roundcubemail-1.6.4-complete.tar.gz
tar -xvzf roundcubemail-1.6.4-complete.tar.gz
cd roundcubemail-1.6.4
bin/installto.sh ../public_html
You can use a similar method for the local Roundcube websites which power mail.customerdomain webmail. You should do this on each of your email servers.
su - roundcubelocal
wget https://github.com/roundcube/roundcubemail/releases/download/1.6.4/roundcubemail-1.6.4-complete.tar.gz
tar -xvzf roundcubemail-1.6.4-complete.tar.gz
cd roundcubemail-1.6.4
bin/installto.sh ../public_html
