We can upgrade ubuntu 11.10 to 12.04 by following the simple steps given below.
1. sudo vi /etc/update-manager/release-upgrades
Then, change the line below from prompt=lts to prompt=normal. To change it, scroll down to the line press the X key on the keyboard to delete each character. Then hit the I key and begin typing the new line. When you’re done, press Esc key, then type :wq to save and exit.
2. sudo apt-get update && sudo apt-get upgrade
3. sudo do-release-upgrade
When ask if you want to continue with the upgrade, type Y for yes.