Does your tablet have an active sim? If so, you can go to Settings > and scroll to the bottom and tap on update. Some older tablets you have to go into About Device and then software update. Without an active sim many devices will not pull the latest update. Depending on the tablet you have you can sometimes use the Software Upgrade Assistant tool by plugging your device into a computer. Google Software Repair Assistant and include your device name/model # and the instructions should come up for you.