How to update Ubuntu server from 9.04 to 10.04

Step 1: install update-manager-core

$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get install update-manager-core

Step 2: Update Jaunty 9.04 to Karmic 9.10
edit /etc/apt/sources.list to replace "jaunty" with "karmic"
$ sudo apt-get update
$ sudo do-release-upgrade

Step 3: Update Karmic 9.10 to Lucid 10.04 
edit /etc/apt/sources.list to replace "karmic" with "lucid"
$ sudo apt-get update
$ sudo do-release-upgrade

To check your server version:
$ lsb_release -a