Following is guide to upgrade from Ubuntu 10.04 LTS to Ubuntu 12.04 LTS.
Make sure you are running a screen session before continuing.
Prepare
Update & upgrade all packages
apt-get update
apt-get upgrade
Install update-manager-core package:
apt-get install update-manager-core
Upgrade distro itself
You are now ready to upgrade to Ubuntu 12.04 LTS.
Open the release-upgrades file for editing by entering the following command:
vim /etc/update-manager/release-upgrades
Verify that the following line is present in the file, and that Prompt is set to lts:
Prompt=lts
Start a screen session:
screen -S distro
Upgrade your Linode to Ubuntu 12.04 LTS by entering the following command:
do-release-upgrade -d
Follow the on-screen instructions to complete the installation process.
When the upgrade process completes, verify that it’s running Ubuntu 12.04 LTS by entering the following command
cat /etc/lsb-release
You should see output that resembles the following:
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=12.04
DISTRIB_CODENAME=precise
DISTRIB_DESCRIPTION="Ubuntu 12.04"