Settings/General/About device/Software update. If there's one available from your carrier for that model phone, you'll get it. If not, you can ty once every 24 hours.
If it's at 4.2, it'll probably update to 4.3 the first time, then to 4.4.2 or 4.4.4, If you get that far, I'd leave it there. Lollipop, or LolliFlop as it's otherwise known, gives you a prettier face and, on most phones, a lot of bugs. 5.0.1 was the bug fix for 5.0. 5.0.2 is supposed to be the bug fix for 5.0.1. I haven't heard of 5.0.3 yet. I may update by 5.6, or I may wait until Google gets its act together (around 12.9 if things go the way they do with most companies like that).