If you have let slip your patching routine with Ubuntu you can find yourself in a position where you can't get any updates or upgrade to the next release.

Backup your existing sources.list file:

sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup

Update the sources to the fallback location:

sudo sed -i -e 's/archive.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

Now you should be able to pull any updates needed before you run your upgrade command.

sudo apt-get update

Then for example you can run one of the following:

sudo apt-get dist-upgrade

or

do-release-upgrade