Well, it depends on what kind of downloads you are talking about. If you are talking about downloading apps, then you can't with your tablet, unless you can update it to Android "Marshmallow", and even then, you also have the possibility of it being a device that the manufacturer has restricted this option on. They do that.
If you are referring to other things, like say music, etc., then you would have to go into the specific app that you are doing the downloads through and change that apps settings to download to the SD card. If it doesn't have that option in that app, then you can't do it.
If you get to upgrade to "Marshmallow" here is how to make the change for apps to download to the card.
How to turn external SD card into...