When an Ubuntu version goes unsupported its repositories get moved to an archive server. If you don’t update the repo origins you’ll get some errors like those:
Err http://archive.ubuntu.com quantal/main i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal/restricted i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal/universe i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal/multiverse i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-updates/main i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-updates/restricted i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-updates/universe i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-updates/multiverse i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-security/main i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-security/restricted i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-security/universe i386 Packages 404 Not Found [IP: 91.189.91.14 80] Err http://archive.ubuntu.com quantal-security/multiverse i386 Packages 404 Not Found [IP: 91.189.91.14 80]
The key to solve this is changing the repo urls from archive.ubuntu.com and security.ubuntu.com to old-releases.ubuntu.com:
sudo sed -i -e 's/archive.ubuntu.com\|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
Then update:
sudo apt-get update
Ref: http://askubuntu.com/questions/91815/how-to-install-software-or-upgrade-from-old-unsupported-release