If this question pertains to the story "The Wife of Bath's Tale", then the hag's appearance changes at the end of the story according to her judgement.
When the knight met the hag, she was ugly and old. She asked the knight to pledge himself to her and she will help him and save him from death. She married him and made him choose if he wanted her to be ugly but loyal and good or young and beautiful but flirty and unfaithful and because the knight gave the hag the freedom to choose for herself, she became beautiful and good.
