OK... There are a couple of options.
If Samsung and your carrier released an update.. You can upgrade easily by going to the settings part of your phone and clicking on check for updates..
If they didn't, then things get trickier... There may be a custom rom that someone has built specifically for your phone. XDA developers is a good place to start. But BEWARE you will have a lot of reading to do... And make sure you follow the instructions carefully, otherwise you may have a bricked device on your hands and a bigger pile of work to get it working again.