You mean an official Samsung update or a custom ROM?
If you want an official Samsung update, you use OTA or Samsung Kies on your PC. If they say it's not available, it's not yet released for your device.
Custom ROM's are available at XDA, but remember that if you use a custom ROM, which is not made by Samsung, you lose TouchWiz features like MultiWindow, SmartStay, AirView, etc.
Some Samsung official firmware are available at Sammobile.com, but you need to know your device's specific build number. Flashing the wrong firmware will soft brick the device. It won't matter if it's the same phone model, it should be a compatible build number, so I can't give you a link without you providing that info.