The On5 doesn't have this option. I can give you the instructions on how it would be done on a device that allows for it. It would have to have Android "Marshmallow" and be unrestricted by the manufacturer, however I have run into this device many times and it doesn't work for it. I would say the manufacturer has put the restriction on that particular device. Here is the way to do it, just so you have it.
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...