Open up your terminal and add the following lines to your /etc/apt/sources.list file as root.
deb http://http.us.debian.org/debian/ lenny main contrib non-free
deb-src http://http.us.debian.org/debian/ lenny main contrib non-free
I also recommend removing (or commenting out) any cd repositories listed in the sources.list file as after a basic install you usually don't need the cd anymore. If you installed using the first Debian install CD the line to remove will look like follows:
deb cdrom:[Debian GNU/Linux 4.0 r2 _Etch_ - Official i386 CD Binary-1 20080102-13:35]/ etch contrib main
Save the file and then update the package manager:
# apt-get update
The final step is actually upgrading your computer to Debian Lenny. To do this we need to do a distribution upgrade. To do this type the following line in your terminal:
# apt-get dist-upgrade
NOTE: This process could take anywhere from 20 minutes to an hour depending on how many programs are installed on your computer, how fast your internet is and the specifications of your computer. I do not recommend doing this if you have a dial-up connection, instead get a hold of the latest weekly built Lenny install CD (cd 1 only).
