Create a backup and download the newly created .zip file. Make sure you save it somewhere safe, such as your Desktop or Downloads.
The latest version of 3CX V16 is the 188.8.131.52 build. "3CX Version 16, Update 8 FINAL" is the required version to install before the Version 18 Upgrade is available for download.
Watch the email address you have configured for notifications, as failure notifications will go here.
a. Updates section of 3CX Management Console
apt update && apt upgrade -y && apt autoremove --purge now
shutdown -r now
You should then get an Update Successful email.
When loading the web client after update, it may show "Disconnected. Waiting for network." Clear your browser cache and you may need to restart the underlying system for the new version of the web client to show.