Well it is going to depend on what version of Android your tablet is running. If it is running "Marshmallow", and the manufacturer hasn't restricted the option to do this, you can set the SD card as your default storage by turning it into "Internal" storage rather than "External".
However, if you are running anything lower than "Marshmallow", or aren't able to upgrade to it, then you cannot do this.
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...