Well it is going to depend on what version of Android you are running. If you are running anything less than "Marshmallow" then it probably won't work. If you can upgrade to "Marshmallow" then the following instructions should help. Unless the manufacturer has restricted that option on that device, which sadly can happen.
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 “three-dot menu“ (top-right)...