How does a "bad parent" redeem themself?

Here's what I mean (first of all, this is not about me nor anyone I know. I just think it's a psychological question that I'd be interested to see all the different answers to) :

Someone is a "bad parent" (which could be anything from slightly mean to abusive, take it however you'd like), and one day, they realize they were wrong in their behavior, and they decide they must change. How does this parent fix their reputation with others, and with their children? How does this parent redeem themselves to their children?