upgrade to Ubuntu 10.10 From Ubuntu 10.04

Ubuntu 10.10 Maverick Meerkat Beta is released! If you have ubuntu 10.10 Lucid Lynx or older version of ubuntu installed and you want to upgrade to Ubuntu 10.10 Maverick Meerkat, you can do it by following these instructions.
- Desktop Upgrade:

So to upgrade from Ubuntu 10.10 or older on a desktop system, press Alt+F2 and type in update-manager -d into the command box. Update Manager should open up and tell you: New distribution release '10.04' is available ( See screenshot bellow).
Before to upgrade remember that is important to update your system first then click Upgrade and follow the on-screen instructions:

1- Press Alt+F2 and type in update-manager -d

2- Now follow these steps :

First check for updates, then Install them, and finally click Upgrade and follow the on-screen instructions.

Server Upgrade:

To upgrade from Ubuntu 10.04 LTS on a server system:

1- install the update-manager-core package if it is not already installed:
sudo apt-get install update-manager-core
2- Edit /etc/update-manager/release-upgrades and set Prompt=normal;
3- Launch the upgrade tool with the command
sudo do-release-upgrade -d

and follow the on-screen instructions.

Source : Upgrade Ubuntu