Release: The following signatures were invalid: KEYEXPIRED

I’d recommend trying to remove the key, and let the GPG key get installed automatically, when you are prompted for the new key. Depending on the strict of the output of the error, for me it was:

W: GPG error: http://download.opensuse.org ./ Release: The following signatures were invalid: KEYEXPIRED 1489340837
sudo apt-key del 1489340837
sudo apt-get clean
sudo apt-get update

Upgrading Ubuntu 12.04 to 14.04 when getting 404 not found for repo links

A customer had this issue with their very old Ubuntu 12.04 machine.

Use this link to generate a new sources.list, and populate your /etc/apt/sources.list with that detail. Backup the current sources.list you have first.

https://repogen.simplylinux.ch/

Then to update with the new repo, use:

apt-get update
apt-get dist-upgrade

Please note that going from Ubuntu 12.04 to 14.04 isn’t something I’d recommend you do if you don’t know what your doing, certainly expect the possibility that the box might not come back up.

Try populating your /etc/apt/sources.list with this:

#——————————————————————————#
# OFFICIAL UBUNTU REPOS #
#——————————————————————————#

###### Ubuntu Main Repos
deb http://uk.archive.ubuntu.com/ubuntu/ precise main
deb-src http://uk.archive.ubuntu.com/ubuntu/ precise main

###### Ubuntu Update Repos
deb http://uk.archive.ubuntu.com/ubuntu/ precise-security main
deb-src http://uk.archive.ubuntu.com/ubuntu/ precise-security main

Failing that, delete the sources.list and put this instead in it:

#——————————————————————————#
# OFFICIAL UBUNTU REPOS #
#——————————————————————————#

###### Ubuntu Main Repos
deb http://uk.archive.ubuntu.com/ubuntu/ trusty main
deb-src http://uk.archive.ubuntu.com/ubuntu/ trusty main

###### Ubuntu Update Repos
deb http://uk.archive.ubuntu.com/ubuntu/ trusty-security main
deb-src http://uk.archive.ubuntu.com/ubuntu/ trusty-security main