What phone do you have, what country/carrier do you have? And what operating system are you on? Go to settings - about phone - scroll down to Android version.
With some info, someone would be more likely to help. Good luck!
Is it a Samsung Tab? I have one and know that it cannot be updated. If it is an older tablet, most likely (bordering on most definitely) it cannot be updated mainly because of its hardware. But we need more info to help you. In the settings there is also a Hardware Version that is the info we also need.
What everyone is getting at is you need to find a specific method to "root" your device. Then you need to find a method to install a custom ROM (OS) on your device. Then you need to find a custom ROM made specifically for your device that you trust will work correctly. Without knowing much about your specific device it will be nearly impossible to do all of that, if those methods and ROM's even exist. Keep in mind that if you try to do this with files or methods for a different device you could end up "bricking" your tablet (making your device useless).
Android isn't like Windows where you can just install a generic version on your PC. You need to have a version of Android that's made for your specific hardware for it to work correctly. Many times the companies that make the hardware don't make new versions so you have to hope that there's someone out there that made their own version for you. If no one has done that then there's no way to update the OS on your device.