Welcome to Android Central! In order to update, there has to be an official system update provided by the manufacturer. It's pretty unlikely that the manufacturer of your tablet would have done that, since it sounds like an old budget device. If you go to Settings>About Tablet>System Updates and tap Check for Updates, and it tells you the device is up to date, then there are no more official updates for the device.
Your only other option would be to see if someone developed a custom ROM for that device based on Marshmallow. If so, then you'd have to root the device in order to install it. It's not something for beginners -- you can try searching the forums at XDA-Developers.com to see if there's anything for your specific device.