That's a low-end phone released in 2014, so you are almost certainly not going to get any more updates for it. It looks like the most recent firmware for this device (any of the several models) is Android 4.4.4.
You can't just download and install a more recent version from somewhere. Android (like any mobile OS) has to be built for the particular model of device. So unless the manufacturer makes an update (in which case "check for updates" in the system settings will tell you) your options are to live with it or to root the device and (a) find a "custom ROM" that someone else has built for this particular model or (b) build one yourself (which will require a lot of learning). For most people "live with it" is the answer, I'm afraid.
If you want to try the custom ROM route, do your research first: make sure you understand what you have to do step by step, make sure that a suitable custom ROM actually exists for your model of device, and make sure you know how you will undo it or get your phone working again if something goes wrong. And I'm very serious about "your model of device": I don't just mean "Galaxy" or "Galaxy Mega" or even "Galaxy Mega 2", I mean the particular model of Mega 2 you have (e.g. SM-G750F or SM-G7508Q) - trying to install a ROM for a different model will not work.