Well, if you are running the Android version it came with, "Lollipop", then you cannot do that. However, if you have upgraded to, or can upgrade to "Marshmallow", then you may be able to use the instructions below to make the change. Note, some manufacturers have blocked this option, so there is no way to know if it is blocked on yours until you try.
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", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.