updates - How to resolve "No suitable download server found" when updating?

I am a new user to Ubuntu 12.04. I have already connected to the Internet (without any firewall or proxy) and can browse the web. But whenever I am trying to update Ubuntu or trying to select the mirror for my location (Singapore) it just gives this error:

NO SUITABLE DOWNLOAD SERVER FOUND. PLEASE CHECK YOUR INTERNET CONNECTION.

What can I do to fix this?

To update by terminal ,open Terminal by ctrl+alt+T

Type

 sudo apt-get update

It will ask password, give password.
Packages will be automatically updated.

To Update by Synaptic

open synamptic , Click on reload.... It will show you list of packages that are available in new version. So are newbie to ubuntu, install synaptic by

Open Terminal,
type
sudo apt-get install synaptic