Ubuntu: apt-get update gives 404 Not Found error

If you recieve “404 Not found” during a apt-get update / apt-get upgrade, the problem can be one of two things:

1)  Your Ubuntu installation is no longer supported.
You can check this by comparing the output of the command:
against the list of Ubuntu releases here:
https://wiki.ubuntu.com/Releases (Notice “End of Life” date)

If your release has reached end of life, you can do upgrade to a new release by following the guide here:
http://www.ubuntu.com/desktop/get-ubuntu/upgrade

2) Temporary problems
The mirror(s) you are using can have temporary problems, in such case you should simply try again later.