If you have tried Settings > Software update and that hasn't worked, try downloading Smart switch on your computer and seeing if that will update it. There is a setting in Smart switch that will make it look. If THAT doesn't work, google for a local UBREAKIFIX store in your area. Take it to them and have them do a reset on it. They will load it with the latest OS for your phone. Takes about an hour or so and usually is free. MAKE SURE THAT YOU HAVE EVERYTHING BACKED UP. Either on Samsung or through smart switch.
Or google ODIN and run a manual installation.