PureOS has gone under some big changes over the last year or so. In addition to the work being done on PureOS to make it convergent on more devices, we've also introduced a new stable version of PureOS called "Amber". We still continue to update Amber although not as frequently. Important updates, like security updates and other important fixes come in now to two dedicated suites of packages called amber-updates and amber-security.
These ought to have come into your system automatically in a package called "base-files" but if they didn't, you can add these suites via the Software tool or on the command line.
Simply add these two lines to the /etc/apt/sources.list file;
deb https://repo.pureos.net/pureos/ amber-updates main deb https://repo.pureos.net/pureos/ amber-security main
Updating to the latest Amber is a good idea before moving to Byzantium.
If you're ready to move to Byzantium you can simply add the repo via the GNOME Software tool, or you can edit your /etc/apt/source.list file.