Otherwise you can wait and hope that some good christian.
Make her your concubine, and then you should get a decision to change to her religion. For more information, check out my tutorial playlist: Probably the quickest way to change is raid down some holdings until you grab a female catholic prisoner. This is a guide on how to maximize the religious conversion of territory in Crusader Kings 2. Religious Head - Crusader Kings II Wiki.Religious conversion is the act of a province or character switching to another religion.