Well, it is going to depend on what Android version the tablet is running and if the manufacturer has restricted this option or not.
First, the tablet would have to be running "Marshmallow" to allow for making the SD card "Internal" rather than "External" storage. If you are running, or can install, "Marshmallow" then try the instructions below.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the...