Well that phone comes originally with Android "Lollipop" which does not allow for making the SD card "Internal" storage. You need at least "Marshmallow" to do that.
Now if you can upgrade that phone to "Marshmallow", and the manufacturer hasn't restricted the ability to change the card to "Internal", then the instructions below are how to change it over.
NOTE: You will need to start with a blank SD card. Why? Well because when it is formatted and encoded to make it "Internal" storage, it is wiped. Also, the data on the card then will only be readable by that device.
How to turn external SD card into internal storage on "Marshmallow".
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.