The following steps below from Ubuntu will guide you in terms of successfully upgrading your Ubuntu system from Natty Narwhal 11.04 to Oneiric Ocelot 11.10.
To upgrade from Ubuntu 11.04 on a desktop system, press Alt+F2 and type in “update-manager -d” (sans quotes) into the command box. Update Manager should open up and display following message: “New distribution release ‘11.10’ is available. Click Upgrade and follow the on-screen instructions”.
To upgrade from Ubuntu 11.04 on a server system, follow the steps listed below: Step 1: Install the update-manager-core package (if it is not already installed). Step 2: Use following command sudo do-release-upgrade -d to launch the upgrade tool. Step 3: Follow the on-screen instructions. Note that the server upgrade is now more robust and will utilize GNU screen and automatically re-attach in case of dropped connection problems, for example.