Unfortunately, with that device, you can't set the default download for everything as the SD card. Only devices running "Marshmallow" have that option, and only if the manufacturer hasn't also restricted that option on a device.
You do have the option of setting specific apps to download or save to the SD card, and you can move some apps to your card. Also, room can be freed up by moving photos and videos to the card (they take up a lot of space).
Otherwise, the only option you would have to make the SD card the default place for downloading all, including apps, would be to root the device. Which although the area is for phones, the principle is the same since both are running versions of the Android OS. I suggest, if you want to...