UBUNTU 10.4 : Lucid Lynx

Until now ( April 10 2010) Ubuntu 10.04 Lucid Lynx still in Alpha release.
The final stable will be released on April 29, 2010.
Well if you intend to try Lucid Lynx., you can download the Alpha 2 release officially from here :

Ubuntu Desktop and Server
Ubuntu Server for UEC and EC2
Ubuntu ARM
Kubuntu Desktop and Netbook Remix
Xubuntu
Edubuntu
Mythbuntu

For upgrading from your current Ubuntu (9.10 or 8.04 LTS) just press alt+f2 on desktop and type in "update-manager -d" (without quotes).
Then you will get notify about the new distribution of Ubuntu 10.04.