While being logged in as root, you’ll have to edit the file /etc/update-manager/release-upgrades and set the value prompt to normal.

vi /etc/update-manager/release-upgrades
set 
prompt=normal

The update process will update your server to every single releases of Ubuntu (200MB download for each step and a restart required)

apt-get update
apt-get upgrade
apt-get install update-manager-core
do-release-upgrade -d

You are now on Ubuntu 10.10 Maverick Meerkat run one more time:

do-release-upgrade

You are now on Ubuntu 11.04 Natty Narwhal run one more time:

do-release-upgrade

to finally enjoy using Ubuntu 11.10 Oneiric Ocelot!

Ubuntu version names