Can I get an update that isn't for my phone yet? I'm on 4.1.2 right now and want to update to the latest version (I believe it's 4.2.3). When I hit 'Check for updates' in settings > about phone > software update it says there's no update avaliable.
Just because Google released a new version of Android doesn't mean that Samsung (or any other manufacturer) has implemented it for any or all of their devices or that your carrier has made it available OTA.

Samsung does not appear to have made any announcement regarding if or when the Stellar may receive an update to JB 4.2.

You may also wish to keep an eye on When will my device get the Android 4.2 update (Jelly Bean)?