I'm not sure what you're trying to do. If you want to edit the file, you need to use an editor. So as root you would do something like
nano /etc/apt/sources.list and then modify it to point to the repositories you want.
If you want to update apt, you run
apt-get update. If you want to install software you run
apt-get install softwarename.
Here's a good
link to how to work with Debian.