Connect to your computer and update via iTunes.
You need iTunes versio 10.7
Are you sue you have a 3GS? A 3G can only go to 4.2.1
Rephrasing your question from another post is not going to get you an answer that you want to hear if that is not the correct answer. For you to be using an iPhone 3GS that was purchased in the US to be used in India, it had to be hacked. You were told that in the other thread. If you attempt to update the phone, it will be locked back to AT&T. Do you understand that?
Get it officially unlocked through AT&T: http://www.att.com/esupport/article.jsp?sid=KB414532&cv=820#fbid=3xy94pWGl76
Then use Google to learn how to Restore it which will update it and clear the lock.
There is no other way to do this.
- Be sure you have iTunes 10.7 (11 will be released soon)
- Plug in your iPhone
- Go to the iPhone summary screen
- Hit "update"
If the update fails
- Go on your search engine and type "iPhone 3GS firmware GSM" or "iPhone 3GS firmware CDMA" and download the latest firmware from there.
- Place the downloaded firmware on your desktop or in your downloads page
- Go back to the summary page
- Hold Shift and click update
- Find the firmware in your saved location
- A question will pop up asking if you want to install the firmware