I don't need a list of similar questions, I want an answer to my question.
The latest iOS version available for iPhone 3GS is 5.1.1. Unless Apple develops the new 6.0 version for 3GS model, the future regarding this update is uncertain. The device might be able to run the OS but it will be very limited due to the new hardware changes in the new iPhone 5.
To upgrade IOS on your Iphone 3GS you can follow the video http://www.youtube.com/watch?feature=player_embedded&v=Kj91zkMrxso . Make sure whether your Iphone 3GS get unlocked using permanent unlocking solution. Because if you unlocked your Iphone 3GS using temporary unlocking solution it will get locked to its network when you update or upgrade it. If not unlock your Iphone 3GS using remote unlocking service from the site Prounlocking.com ,which is a permanent unlocking solution. Then you can upgrade IOS 5 on your Iphone 4 it won't get locked again.