Well, first you need to be running at least Android "Marshmallow" and second you must have a phone that the manufacturer hasn't restricted this option on. If any of the steps in the instructions below don't show or work on your phone, then you probably have a device the manufacturer has restricted.
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 “three-dot menu“ (top-right), now select...