First, you will need to be running Android "Marshmallow" or newer for it to work. Second, you will need the instructions I am including below. And third, you will need a phone that hasn't had this option restricted by the manufacturer.
NOTE: If any of the options in the instructions doesn't exist on your device, or any of the steps don't work for you, then the phone was restricted by the manufacturer.
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...