Now Ubuntu Lucid (10.04) LTS(Long time support) beta1 has released and the final stable version will be released on April 29, 2010.Here is how to upgrade from Ubuntu 9.10 or 8.04 LTS.

To upgrade from Ubuntu 9.10 on a desktop system:

press Alt+F2 to launch the command box,and type in update-manager -d. Update Manager should open up and tell you: New distribution release ‘10.04’ is available. Click Upgrade and follow the on-screen instructions.

To upgrade from Ubuntu 9.10/8.04 LTS on a server system:

install the update-manager-core package if it is not already installed;
edit /etc/update-manager/release-upgrades with your favourite editor:
for 9.10:
set Prompt=normal
for 8.04 LTS
set Prompt=lts
Finally,execute sudo do-release-upgrade -d command to launch upgrade tool and follow the on-screen instructions.

Hope this help!