Well, it depends on what you want to set it as default for.
If you want to set it as the default location for photos you take, downloads (other than apps), etc., then you would just need to change the settings in the particular app using, in this case the camera, gallery and browser.
However, if you are wanting to download and run apps from the SD card, that is totally different. You will need 3 things. 1. Your phone will have to be running, or able to upgrade to, Android "Marshmallow". 2. You will need the instructions on how to make the change, which I am including here. And 3. Your device will have to have 'not' had this option restricted by the manufacturer. Which sadly does happen.
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 “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.