depending on your country and network there may be an update available to you officially.
You need to download kies from samsung's website and install onto your home pc - then plug phone in to pc and launch kies. This will tell you if theres an official update available for your phone.
Alternatively you can download a firmware find links on here or on xda forum and install via odin, again find links on here or on xda for instructions.
This is the unofficial way, but gets you the updates quicker.