To put it simply, there is no definate spell to change gender.
There are, though, various ways to experience being the opposite sex, although you would return to base-line afterwards. These techniques involve the use of drugs and visualizations, as well as lucidity within dreams & astral projection. Prolonged stay within those methods would no doubt cause psychosis if you simply cannot shake the idea from your mind. Keep in mind there are always sex-change operations, and hormonal therapy.