According to FAQ yes you can and JW will acknowledge your basic human rights to change or leave your religion without any consequences.
But the elders book says No you can't just leave resign ,your either disfellowshiped or dissociation which basically means the same you will be punished for resigning .
But to the public or a study it's looks like you can leave at any time but as we all know that the org talks out off both sides of its mouth,and has one side for the public and another for cult members.