How can a woman atone for having engaged in anal sex with her fiancé (who is now her husband), knowing that it happened only once and that she was unaware of its prohibition at the time?

Engaging in anal intercourse with one's wife is a grave sin, and the Prophet, peace and blessings be upon him, cursed those who commit such an act, saying: "Accursed is he who has intercourse with a woman in her anus." He also said: "Whoever has intercourse with a menstruating woman, or a woman in her anus, or consults a soothsayer, has disbelieved in what was revealed to Muhammad." The spouses must repent to Allah Almighty for this abomination and increase in righteous deeds, for by doing so, sins are erased. Allah Almighty says: "But whoever repents after his wrongdoing and makes amends, indeed, Allah will turn to him in forgiveness. Indeed, Allah is Forgiving and Merciful." What occurred between you does not make her unlawful to you, as there is no Shar'i evidence for that.