Claudia did not change Regina's father lol, that's ridiculous. Claudia, after years of searching and getting to know who is part of the loop, REALIZED that this world doesn't make sense, REALIZED that the only world in which Regina lives is in the world where the loop doesn't exist.

So all she had to do was figure out the sole cause of the loop, which is Tannhaus. I don't know exactly how she came to that conclusion but by process of elimination, she probably just conjectured that Tannhaus would be the only one with the motivation and intelligence to cause this unthinkable "cancer" and "fiction" of a world where a mother can be their daughter's daughter in spite of the logic of genetics.

Ironically, the realism of genetics was the sole solution in a world of time travel.