Trump famously lost on January 7 when Mike Pence officially declared Biden the winner. There is no going back.
However there is a relatively recent example of allegeded voter fraud in the Kennedy/Nixon election contest. Kennedy had help from the famous Mayor Richard J. Dailey. He helped John Kennedy in Cook County ( Chicago) fraudulently (allegedly) win Cook County therefore Illinois. And according to Robert Caro LBJ biographer, Johnson known for his skill in getting elections outcomes, swung Texas to the Kennedy/Johnson ticket. Without Illinois and Texas, Nixon would have been president. It was a different time then, even Richard Nixon graciously conceded the Presidency to John Kennedy.
Getting the right vote count is the basis of our democracy. We have a constitutionally mandated state election process to get the vote out. We need to support any efforts at the state level to get an accurate count.