upgrade to debian instable

As a beginner on commande lines, I failed at the very first step to upgrade my stable version (etch) to unstable version before installing Kdenlive.

typed "etc/apt/sources.list" as "su" on agraphic mode on gnome : "permission denied"appeared.

As single user, from grub I had the same same answer.

So, I don't know how to add the following lines "deb-src ....  deb http ....."

What is wrong ?

Some please can tell me ?