There are two options to solve this problem i.e command line and GUI.
Go to the terminal via Applications>> Accessories>>Terminal
Type the following commands:
sudo sources.list sources.list20110328A_Backup
sudo gedit sources.list
You will get source list gedit dialog box.
Replace all in with us in the URL and save and quit the dialog box.
Then go to update manager and install updates.
Graphic User interface:
System -> Administration -> Synaptic Package Manager -> Setting ->
Repositories -> Ubuntu Software
Look for “Download from”, select main Server (or select ‘Best Server’
from ‘Others’ ), close
File -> Exit