First decide which version of Debian you wish to upgrade to. The most common versions available are:
v3.0 Woody 2002
v3.1 Sarge 2005
v4.0 Etch 2007
v5.0 Lenny 2009
v6.0 Squeeze 2011
"wheezy" - not yet released
Open the /etc/apt/sources.list file with your favorite text editor and modify any references to the code name of your current version so that they read the code name to the version youre upgrading to. For example: if youre upgrading from v4.0 Etch to v5.0 Lenny you would replace the word "etch" with "lenny" on every line. Your /etc/apt/sources.list file would look similar to the following after modifcation.
#####
deb http://ftp.us.debian.org/debian/ lenny main
deb-src http://ftp.us.debian.org/debian/ lenny main
deb http://security.debian.org/ lenny/updates main
deb-src http://security.debian.org/ lenny/updates main
deb http://volatile.debian.org/debian-volatile lenny/volatile main
deb-src http://volatile.debian.org/debian-volatile lenny/volatile main
#####
To begin the upgrade after your version has been chosen and added to the /etc/apt/sources.list file, execute the command "apt-get update" followed by "apt-get dist-upgrade -y" then wait for any prompts that require your attention. When the upgrade is complete execute the "apt-get dist-upgrade -y" command a second time to ensure the upgrade was successful. You should see the following on your screen if the upgrade was successful.
# apt-get dist-upgrade -y
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Perform a soft reboot using the "shutdown -r now" command so you can boot into any new kernels that Aptitude may have installed during the upgrade.
