What is the ruling on a girl separating from her first fiancé to become engaged to another who is better in character than him?

The engagement is merely a promise of marriage, and it is permissible for either party to dissolve it for a valid reason. If a girl discovers a flaw or deficiency in the suitor, she has the right to dissolve the engagement. However, it is not permissible for a Muslim to propose to a woman who is already engaged to another Muslim, or to corrupt a woman against her suitor if the suitor has been given a clear answer. If the girl learns that a more righteous suitor will propose to her without instigation from the second suitor, or if the second suitor is unaware of the first engagement, then there is no harm in her dissolving the engagement. She should perform Istikhara (prayer for guidance) regarding the entire matter and not rush into accepting any suitor until she has inquired about his religiosity and character.