I have slandered many people in my youth and I was wondering how I can repent for the ones I forgot I slandered?

If you remember those people contact them and make an apology and explain them In’Sha’Allah they will forgive you if not possible to contact give sadaqah on behalf of them and pray namaz radd e mazalim
In’Sha’Allah Allah will forgive. In’Sha’Allah from now refrain from it