How do i upgrade my Iphone 3gs to iOS 4.3?? bit of a novice and really stuck. Someone told me it would do it automatically but its not when i attach to i tunes

The only signed firmware, available for your phone(if a 3GS) is iOS 5.0.1. Connect your phone, iTunes running, on the Summary Pane, hit the "Check for Update" button.

Version of iTunes on your computer? You need iTunes 10.5.3. Sure you have a 3GS? Settings>General>About>Model...what does it say?

http://support.apple.com/kb/HT3939

You can't upgrade the 3g past 4.2.1, you're out of luck. Sorry.

Your only next step is to upgrade the phone to a newer model. Your 3G is at the end of its line, and can no longer be updated. More and more apps will no longer work with 4.2.1.

I regretfully retract my previous comment on how to downgrade the iOS firmware version. I was not aware of the issues he/she was experiencing.

Downgrading the iOS is not supported by Apple, is likely to create more problems (iOS - modem baseband version mismatch), and won't be useful to the OP as his problem is apps requiring iOS 4.3 or later, not slowness of the phone. Downgrading the iOS will only make his problem WORSE.

AND, as you should know, discussing or recommending unauthorized modifications of the phone violates the forum Terms of Use.

Violator.

You haven't violated anything. Downgrading iOS is forbidden by ToU, but you weren't the one who suggested that.

Under the title of the post, there is a line showing who the post is responding to. stevejobsfan0123 was not addressing you.