Natty Narwhal Alpha 2 | Ubuntu
To upgrade from Ubuntu 10.10 on a desktop system, press Alt+F2 and type in "update-manager -d" (without the quotes) into the command box. Update Manager should open up and tell you: New distribution release '11.04' is available. Click Upgrade and follow the on-screen instructions.
To upgrade from Ubuntu 10.10 on a server system: install the update-manager-core package if it is not already installed; launch the upgrade tool with the command sudo do-release-upgrade -d; and 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 e.g. dropped connection problems.
To upgrade from Xubuntu 10.10, follow the instructions above for Ubuntu 10.10.