The repo structure should be permanent, but if there are any changes they will be pointed out in the release posts, found in the Community.
Follow the instructions below to stay with the current stable release, regardless of changes in versions.
How you get to Software Sources depends on which release of Ubuntu you are using. If you also see one for Security, remove that one as well. Try opening Software Sources, go to the Authentication tab, and remove the keys for Extras and Ubuntu Archive. Copy the numbers to the following command (you can use the full code rather than just the last 8 characters) and try adding them again.If you also see one for Security, remove that one as well. When you accomplished the earlier steps, did you also remove the key for Extras in the Authentication tab of Software Sources before running the above command?But sometimes you may encounter the following situation: a KEYEXPIRED message.root# apt-get update && apt-get upgrade Get:1 xenial-security In Release [94.5 k B]Hit:2 xenial In Release Get:3 xenial-updates In Release [95.7 k B]Hit:4 xenial-backports In Release Hit:5 https://packages.cisofy.com/community/lynis/deb stable In Release Get:6 xenial-updates/main amd64 Packages [373 k B]Ign:7 xenial In Release Get:8 xenial Release [2,309 B]Get:9 xenial [287 B]Get:10 xenial-updates/main i386 Packages [368 k B]Get:11 xenial-updates/universe amd64 Packages [319 k B]Get:12 xenial-updates/universe i386 Packages [316 k B]Err:9 xenial The following signatures were invalid: KEYEXPIRED 1471427554Fetched 1,566 k B in 0s (2,003 k B/s)Reading package lists…Done W: An error occurred during the signature verification.