Ask'n for help is not bugging ! - if you had of followed directions from above I would have already so advised -
1st backup the file, Murphy's law always applies - !
Now fire up the text editor gedit with the required elevated privilege to edit a system file; opening to that file.
sudo cp /etc/apt/sources.list /etc/apt/sources.list-26apr2014
Insert the terms ubuntu precise at that correct line and field(s).
gksudo gedit /etc/apt/sources.list
Save the file and exit back to the terminal.
Make sure now that all is good:
All is good ? .. is USC now happy happy ?
sudo apt-get update
sudo apt-get upgrade
ain't nothing but a thing