Unfortunately that isn't possible. Only phones with Marshmallow have the ability to use an SD card as storage, as that OS allows for making the card Internal rather than the standard external.
The only thing you can do is download directly to the phone, and then move things over to the card. However, not all apps allow for moving to a card. Also I would really suggest moving/copying any personal stuff on the phone and the card to a cloud account or a computer. This can free up space for you and save you from losing things.