"Neha eats her apple."
Translation:नेहा अपना सेब खाती है ।
It can be written using the oblique case as well, by including the postposition को.
नेहा अपना सेब खाती है। No postposition -> Object in direct case.
नेहा अपने सेब को खाती है। Postposition -> Object in oblique case.
Note that the postposition isn't omitted when the object is a proper noun or a personal pronoun.
As I understand it, upna is one's own as in 'my own', 'her own', 'his own', uska is just his or hers, unka is theirs. So if Neha was eating someone else's apple and not her own, you might say 'uska seb', if she was eating their apple, it would be 'unka seb' or if something of the female gender like bread it would be upni roti (her own bread) 'uski roti' (his or her bread) or 'unki roti' (their bread). At least that's how I understand it. (I don't think upka is a word. Aapka or aapki means yours.) Do please correct me, someone, if I'm wrong.