Old Ubuntu

If you can not update anymore your Ubuntu release, you may have to change the repository from which you download packages.

You can edit /etc/apt/sources.list and change all http://whatever.ubuntu.com/ubuntu to http://old-releases.ubuntu.com/ubuntu

Sometimes, it may be necessary to add also the updates channel. For Trusty for instance:

deb http://archive.ubuntu.com/ubuntu trusty main universe restricted

deb http://archive.ubuntu.com/ubuntu trusty-updates main universe restricted