This may be a late response but just in case you're still looking for an answer to this question.
Settings > About phone > Software Update (make sure you are connected to the internet)
Have you tried these steps? If yes, and it always result to "Your system is up to date." it is because the manufacturer of your phone doesn't actually support a system update to the unit of your phone. And that's a sad truth to a mid-range Android phone and lower.
You can actually check online if your phone will get any software updates.
First, check the brand of your phone and then the unit. But I bet you already know that. Also check the latest current version of the system. Then search it on google the supported version of your phone.