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.