Q: software update problem iphone 4

dear phone users:

good day !

i am using an iphone 4. my current os is from 4.3.2 & i am willing to upgrade it to 6.1.2. when i intend to upgrade itune shows-

"The iPhone software update server could not be contacted. Make sure your network settings are correct and your network connection is active, or

try again later."

i already goggled a lot & tried all craps like host file corrention + network settings but no result. pls help

using macbook & os is withdows 7