How to Upgrade a Galaxy Tab A Model SM-T550 to the Latest Android
If you are lucky for a tablet you can get one major Android version upgrade and security updates up to 2 years after the device has been released to market. After that time span usually do won't get any official updates anymore.
This situation is sad. Samsung is one of the manufacturer who release updates for a long time for it's premium devices.
If you want to get a newer Android version you need to unlock/root the device and install a custom ROM (if there is a working one available for your device).
The only manufacturer on the market who provides updates to smartphone and tablets for a long time is currently Apple.