You assume a different form of your sex. When you cast the spell, choose what aspect will change. You can change your private parts, or your whole body depending on how focused you are on the spell.
Change Appearance. You transform your appearance. You decide what you look like, changing your body shape entirely if its to your liking, however your height, weight, facial features, and distinguishing characteristics, if any, will stay the same. You also can't appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can't use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.
This basically goes to everyone who just want to change their sex, have fun.