For example, Siri is compatible on an i Phone 4s but not on an i Phone 3GS, this is simply because it does not support the system requirements (or because Apple wants more money).
There are 2 ways to update to i OS 6; through i Tunes and through your devices software update.
Updating through i Tunes is as simple as plugging your device into i Tunes and clicking the “Check for Update” button in the devices summary pane.
If you are not using i OS 6 already, i Tunes will pop up a little window saying an update is available.
Apple decided to support the i Phone 3GS with i OS 6, although with the release of the i Phone 5 the hardware is now discontinued.
Some features are limited on the older hardware, however i OS 6 is available for OTA or i Tunes-based update for 3GS owners.
;( I could be wrong, but I think you're stuck on i OS 4.
Sometimes this makes more sense for Apple to do instead of merging all of the updates into one, at least for smaller updates.
What you will need is the original IPSW Image from Apple or 4.2.1 or 4.3.3 A Custom Restore Image 4.3.3 IPSW if you have lost your own, one is filed here with a prefix of NO_BB so it will preserve your existing baseband.
While this may seem like a last resort, restoring your phone usually always greatly increase battery life on all Apple devices.
While restoring your phone is easy enough, it seems that some users on Apple forums are not only restoring their devices, but downloading all of their applications directly from Wi-Fi and not using data at all.
If an update is available for your device: To update to i OS 6 without i Tunes, your i OS device has to be connected to Wi-Fi.
The device should also be plugged into a wall outlet (not your computer).