This procedure is best for the server version of Ubuntu:

Edit /etc/apt/sources.list

Change the keyword "breezy" to "dapper".

Comment out any lines starting with "deb cdrom" with the pound sign (#).

sudo apt-get update

sudo apt-get -y dist-upgrade

For more information on the /etc/apt/sources.list file, see "Chapter 2 - Basic Configuration" in the APT HOWTO:

http://www.debian.org/doc/manuals/apt-howto/ch-basico.en.html