HT3939: Identifying iPhone modelsLearn about Identifying iPhone models
Currently Being ModeratedNov 30, 2012 5:30 AM (in response to dyal)
If you really have a 3GS, plug it in, iTunes running, on the Summary Pane, hit the "Check for Updates" button. iOS 6.0.1 will be installed on your phone.
There is no official way to install any other iOS version on your phone, other than iOS 6.0.1.
You need to use the computer your phone normally syncs with. Ignore Simple Apple's advise as you will lose ALL of your data if you follow it.