no more free space after android update
No way to revert back to an old version since you are on marshmallow, Samsung won't allow you to, for security reasons. 

You can get back some space by uninstalling​  bloatwares, but you'll need root. 

Try also moving movable apps to an external memory, works with mine (I also have a J5 8GB), at Settings &gt; Application​ &gt; Application Manager