Upgrade Ubuntu from 9.04 to 12.04

First Verify the version of Ubuntu that you are running:

sudo lsb_release -a
$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get install update-manager-core
Update Jaunty 9.04 to Karmic 9.10
update /etc/apt/sources.list and replace "jaunty" with "karmic"
$ sudo apt-get update
$ sudo do-release-upgrade

if do-release-upgrade is not working just do a 
apt-get upgrade
Update Karmic 9.10 to Lucid 10.04 
update /etc/apt/sources.list and replace "karmic" with "lucid"
$ sudo apt-get update
$ sudo do-release-upgrade

if do-release-upgrade is not working just do a 
apt-get upgrade

From Version 10.04 to 12.04 the Upgrade Get easier:
All you have to do is run the following command:

sudo do-release-upgrade

and follow the prompts.

Useful Links

List of Ubuntu Releases
http://en.wikipedia.org/wiki/List_of_Ubuntu_releases

How to update Ubuntu server from 9.04 to 10.04
http://thanhsiang.org/faqing/node/156

Lucid to Maverick
http://www.ubuntugeek.com/how-to-upgrade-from-ubuntu-10-04-lucid-to-ubuntu-10-10-maverick-desktop-and-server.html

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