Well if you are running the version of Android it came with "Lollipop" then you can't. However if you have or can upgrade to "Marshmallow" then the following instructions should help you.
NOTE: Even with the correct version of Android you still may not be able to as sometimes manufacturers restrict the option.
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...