Server Upgrade:
To upgrade from Ubuntu 11.10 on a server system to Ubuntu 12.04 LTS:
1- install the update-manager-core package if it is not already installed:
sudo su
pwd
apt-get install update-manager-core
2- Edit
vi /etc/update-manager/release-upgrades and set Prompt=normal; [default=LTS]
3- Launch the upgrade tool with the command
sudo su
pwd
do-release-upgrade -d
and follow the on-screen instructions.