How to upgrade Ubuntu 13.04 to Ubuntu 14.04

You are running Ubuntu 13.04. Ubuntu 13.04 has reached it's end of life support on January 27th, 2014, so you will not be able to do install/upgrade anything right now, that's why your apt-get update command is not working.

You should know that if you want to upgrade to 14.04, you have to follow the following upgrade order: 13.04 → 13.10 → 14.04.

1. You first need to edit your sources to use the "old releases" repositories. You can do this by running the following command:

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

2. Now, run the following:

   sudo apt-get update && sudo apt-get dist-upgrade
   

   This will upgrade all the packages to the latest available versions.

3. Now, try the Ubuntu upgrade:

   sudo do-release-upgrade -d
   

   This should work and prompt you to upgrade to 13.10. Do the upgrade, and then go through the same steps again to upgrade from 13.10 to 14.04.

In my opinion, it's just easier to do a fresh install of 14.04 using the ISO file you have, if you know how to do that, that is, and that there's not much data to backup.