Well, first the phone needs to be running at least Android "Marshmallow". Second, you will need the instructions I am including below and third, you have to have a phone that manufacturer hasn't restricted this option on. If they have, then it won't work.
NOTE: If any of the steps are not on your device, or don't work for you, then the manufacturer probably restricted this option on that device.
How to turn the "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...