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"