Adapted from http://askubuntu.com/questions/501976/unable-to-upgrade-from-13-04-to-14-04/502029#502029
1) Edit apt sources to use "old releases" repositories.
sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
But your sources.list file may contain other repositories that have, for me, prevented this process from working.
a) Manually edit /etc/apt/sources.list and comment out these entries
deb-src ...
deb http://archive.canonical.com...
deb http://extras.ubuntu.com...
and comment out any other repositories that were not converted to old-releases.ubuntu.com
b) Change any country specific URLs to 'US' versions
eg. au.old-releases.ubuntu.com to old-releases.ubuntu.com
2) Update packages
sudo apt-get update
sudo apt-get dist-upgrade
3) Upgrade Ubuntu version
sudo do-release-upgrade -d
1) Edit apt sources to use "old releases" repositories.
sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
But your sources.list file may contain other repositories that have, for me, prevented this process from working.
a) Manually edit /etc/apt/sources.list and comment out these entries
deb-src ...
deb http://archive.canonical.com...
deb http://extras.ubuntu.com...
and comment out any other repositories that were not converted to old-releases.ubuntu.com
b) Change any country specific URLs to 'US' versions
eg. au.old-releases.ubuntu.com to old-releases.ubuntu.com
2) Update packages
sudo apt-get update
sudo apt-get dist-upgrade
3) Upgrade Ubuntu version
sudo do-release-upgrade -d