Well, on that device you really can't, as it is running a version of Android that is older (KitKat). However, if you can upgrade the OS to Android "Marshmallow" then you 'may' be able to make the change. The other issue is if the manufacturer has restricted this option or not. They do that sometimes. First you need to upgrade to "Marshmallow" then try the following...
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...