Say you sinned against a fellow Jew. You decide you want to seek forgiveness from that person but then discover that person has passed away. G-d forgives sins against Him but man must seek forgiveness from his fellow man. How does one receive forgiveness from his fellow man posthumously?
Shulchan Aruch here
אם מת אשר חטא לו מביא עשרה בני אדם ומעמידם על קברו ואומר חטאתי לאלהי ישראל ולפלוני זה שחטאתי לו (ונהגו לבקש מחילה בערב יו"כ
If the person against whom he sinned has died, then he brings 10 people with him to stand by his grave and he says: "I have sinned before the G-d of Israel and against So-and-So I have transgressed." (Remah: And our custom is to request forgiveness on the eve of Yom Kippur (Mordekhai)).