All they have to do is let his obvious corruption become a DOJ issue instead of burying it. Game over for Biden.
The time to have done that was last fall. Doing it this close to the election would be a disaster for them, and Joe/Jill would fight it anyway. It would be dragged out until well after the election. Not to mention that leaves them with Kamala....
A huge problem they have is that their party is as fractured as ours. That's the reason Biden was the nominee in 2020, and that's the reason why he wasn't challenged in 2024. They needed someone who was kind of a blank slate and didn't really represent any particular faction of the party.
But if you replace him, there would now have to be a fight as to who the replacement nominee would be. It also would mean that voters themselves would have no voice as to who the nominee would be, which will piss off whatever faction loses.
The time to have gotten rid of Joe would have been last year so that they could have had a normal primary process to select a new nominee. That window has closed for them, so they are stuck with Joe.