How could Democrats replace Joe Biden as their candidate for president?

The rules do not allow for a rerun of a popular election. To become the party nominee, a candidate needs to win a majority of delegates to the Democratic national convention. The number needed is 1,968 and Mr Biden has already gathered 3,894 because he has run virtually unopposed. No one, including delegates to the convention, can force the president to step aside. But if he were persuaded to withdraw (whether by his wife, or by a posse of Democratic grandees), these delegates would become unbound and therefore free to vote for the nominee of their choosing.