Well, your phone should be running Android "Marshmallow", which is the version you would need, so the instructions I am including below should show you how to make the change. That said, please note, some manufacturers have removed/restricted this ability on some of their devices, so if yours can't do this/doesn't show the option, then that is probably the reason.
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"...