How can I atone for a sin I committed against someone who has passed away?

You must repent by regretting the sin, immediately ceasing from it, resolving not to return to it, and restoring any injustices. If the injustice was financial and owed to a person who has passed away, then restore it to their heirs if they are known. Otherwise, give it as charity with the intention that the reward reaches them. If the injustice was not financial, then increase your good deeds, pray for the deceased, and seek forgiveness for them.