i have a iphone 3gs and want to upgrade ios 4.1 to 4.3 or higher verson please tell me the about it.

i have iphone 3GS, having ios 4.1, every time i want to download any application i got an error "this is for ios 4.3 or newer version". so i want to upgrade my iphone's os from 4.1 to 4.3 or higher version. is it possible to upgrade n how? how can i find out that my iphone is locker/jailbroken or not. please help me out.