You could tell them you are sorry and ask them to forgive you but it may just take time for that person to get over being mad. The real lesson here is don't do bad or mean things to people. The fact that you obviously feel bad about it is good in that it shows you have a conscience. I am a very forgiving person but that doesn't mean I still feel the same about someone after they have done something hurtful to me. I don't have room in my life for mean people. Hopefully this person will forgive you and you won't do things like that again.