IOS 5 Update Not Showing on iPhone4

I have a query regarding iPhone4. When I plugged the iPhone to my PC, the iTunes will automatically open. But the iOS 5 update is not showing. The tendency is, I cannot make an upgrade using my iPhone4. I tried to plug in then unplugged the device more than once but the notification to update is still not showing. How can I resolve it? Is there a way that I can manually update it? Thanks in advance.
