Ubuntu 10.04 Lucid Lynx first alpha is released today and you can upgrade your server as well as desktop Ubuntu Linux distribution to this version. This is strongly recommended that don’t install 10.04 Alpha 1 on production servers or daily use desktops. The stable version of Ubuntu 10.04 is scheduled to release on April 29, 2010.
Testers who wants some hands on Ubuntu 10.04 Alpha 1 can upgrade their Ubuntu 9.10 or Ubuntu 8.04 to Lucid Alpha 1.
http://cdimage.ubuntu.com/releases/lucid/alpha-1/ (Ubuntu Desktop and Server)
http://uec-images.ubuntu.com/releases/lucid/alpha-1/ (Ubuntu Server for UEC and EC2)
http://cdimage.ubuntu.com/ports/releases/lucid/alpha-1/ (Ubuntu ARM)
http://cdimage.ubuntu.com/kubuntu/releases/lucid/alpha-1/ (Kubuntu Desktop and Netbook)
If you want to upgrade your Ubuntu 9.10 desktop then press Alt + F2 and then type "update-manager -d" (without quotes). It will show you that a new version of Ubuntu is available. Click on Upgrade to upgrade your current Ubuntu.
To know more about the new features in Ubuntu 10.04, read the blueprint for Lucid. Here is the link for the same,