If you downloaded the 3CX ISO in 2017, then your operating system is probably on Debian 8. To move up to the next update or version you will have to upgrade to Debian 9.

Update 6 includes a script to automatically take you to Debian 9. There is no need to login to SSH or enter any commands in the Linux Shell. We recommend you plan for this…

Continue reading the Original Blog Post.

Source: 3CXS