What should a girl do who prayed to God to marry a specific person, then her situation and perspective changed, and she saw a vision that she would get him, and now she doesn't want him at all? And should she pray to avert this marriage?

The question is unclear. If it is understood that the questioner prayed to Allah to marry a specific person, then changed her mind, and saw a dream about it, then it would have been better to pray for a righteous husband without specifying a person. There is no harm in what she prayed for, whether at first or later. A dream does not necessarily come true, and the matter of marriage is predetermined by Allah. Therefore, she should entrust the matter to Him, have a good opinion of Him, and continue to pray for what benefits her.