You would not be able to do that on a phone running "Lollipop". You have to be running "Marshmallow" to change the card to work as "Internal" rather than "External" storage. Now if you can upgrade to "Marshmallow", it may work. I say may, because there is still the possibility that the manufacturer has restricted the option on that phone. If you can upgrade to "Marshmallow", try the instructions below.
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.