For some reasons it can be neccesarry that you modify the file sources.list. This is the file that debian reads to update his systemfiles or for update some programmes.
Here you can add some new update links, for programmes not standard installed into Debian or to make some changes. Howto do this follow steps below:
Open the konsole
type command => cd /etc/apt (enter)
type command => sudo gedit sources.list (enter)
type your password (enter)
Now do the changes and save. I use here the editor gedit, but you can also use an other editor, whatever you want!