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
Update to the latest Amber 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. Here's what the contents of the file should look like for Byzantium;
deb https://repo.pureos.net/pureos/ byzantium main deb https://repo.pureos.net/pureos/ byzantium-updates main deb https://repo.pureos.net/pureos/ byzantium-security main
Proceed to perform full update. It is recommended to this via terminal command, so that you can see if any errors come up.
This will update a ton of package on your machine, so be patient until it is done. If no errors reported, restart the system and you have Byzantium, the bleeding edge of PureOS.