Well if you are running "Marshmallow", and as long as the phone doesn't restrict the change (Samsung has done that with some devices), then you should be able to do it. The instructions on how are below.
Please Note: You need to start with a 'blank' SD card. The reason for this is that it will be formatted and encrypted so as to only work with that device once it is changed. So anything that was on it beforehand would be deleted.
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.