Well, you have to be running at least Android "Marshmallow" and your phone has to have not had that option removed from it by the manufacturer. It happens a lot. Try the instructions below (for which ever version of Android you have) and see if it will work for you. If not, or an option is missing, then the manufacturer has removed that option.
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 select “Storage”.
2. Select your "SD Card", then tap the...