Show them who you've become. Explain how you weren't the best you back then, saying sorry now is probably late but those who are looking forward to a future where humanity is still valued will be kind to you for your efforts in being a good person. Recognizing your past mistake and showing that you have outgrown it and that you are now a a more compassionate person, is bound to speak for itself. Why not buy them a drink or something? ^^. If you don't want to do it in person, you could at least maybe write them a message? Or a call? They don't know the current you. Maybe they should? Courage :)