how to change the administrator name

I have tried to change the administrator name on a Macbook pro that I recently sold. I used the following:

Rebooted and held the command and s button, pressed enter and ended up with the following at the prompt: bash-3.2#.

I was trying to get to the root directory in order to continue with:

mount uw /

rm /var/db/.AppleSetupDone

shutdown -h now

I saw a video that said this would allow the new user to start fresh and type in his information however the

bash-3.2# does not allow me to