What should I do to atone for my bad assumption about my housemaid and accusing her of theft, knowing that I did not inform her of that, and that I saw her in a dream trying to steal and wanting to hit me?

The basic principle concerning a Muslim is their innocence. It is not permissible to accuse them of theft merely based on suspicion, due to Allah's saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." (Quran 49:12) and the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech."

Therefore, accusation without clear evidence is wrong, and you must repent and seek forgiveness. You should also seek forgiveness from the maid and absolve yourself of all general rights owed to her. It is not necessary to explicitly state the injustice if doing so would lead to harm. As for the dream, it may have no meaning, or it could be from self-talk, or its interpretation might be an encouragement to seek the maid's forgiveness.