What must one who has sinned against another do to repent? Is it necessary to ask for forgiveness from the person himself or not? And what if that person has passed away?

You must seek absolution from anyone you have wronged. If the injustice was financial, return the right to him or his heirs. If you cannot reach them, then donate it with the intention that it is for them. If one of the heirs appears later, pay him his right, and the reward for what you donated will be yours. If the injustice was emotional (such as insult or backbiting), then apologize to him, unless doing so would lead to a greater harm. In that case, seek forgiveness for him and repent to Allah. The Prophet, peace and blessings be upon him, said: "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness before the Dinar and Dirham cease to exist. If he has good deeds, some of them will be taken proportionate to his wrongdoing, and if he has no good deeds, then some of the sins of his companion will be taken and laid upon him." Narrated by Al-Bukhari.