Freshly installed 14.04.5 LTS trusty system, 4.2.0-42-generic, fully updated (using Synaptic). Now when I log on I get the title message. And:
"There is a graphics stack installed on this system. An upgrade to a configuration supported for the full lifetime of the LTS will become available on 2016-07-21 and can be installed by running 'update-manager' in the Dash."
Isn't Synaptic equivalent to or better than Update Manager? Anyway, update-manager repeats the hwe-support-status warning ("New important security and hardware support update"), proposes an "Install..." button (without stating what it'll update), then warns that "Requires installation of untrusted packages". Clicking "OK" apparently just dismisses update-manager (there is no "proceed anyway" option).
How can I get the darn system to update? It may be that there is a security key that needs to be imported, but which one?