Since it keeps getting asked.
If Biden bows out the Democrats CAN LEGALLY PUT ANYONE THEY WANT ON THE TICKET. It is their Ticket they can do with it as they please. No amount of complaining will change that it is legal for them to run anyone they want at any point. They stick to the most popular candidate to have the best chance of winning the election BUT LEGALLY THEY DON'T HAVE TO.
Now some copy-paste from an article I found somewhere.
There is no real possibility of delaying the election. That would require legislation—the House, the Senate, and the president agreeing quickly on new dates—which is politically unlikely, and would be legally awkward when voting has already started. Instead of postponing or redoing the election, the system relies instead on replacing candidates. The rules for doing so are clear, but what will happen under those rules is anything but.
Read: Now what?
Both parties provide in their rules for replacing a candidate. Even at this late date, if a candidate died or became so sick that withdrawing was necessary, party leaders would confer and select a replacement. The process by which they would do this is not set, and is entirely up to them. The leadership in this case are the 168 members of the Republican National Committee, and the 400-plus members of the Democratic National Committee. (The RNC allows for the possibility of reconvening its national convention, but doing so this late seems unlikely.
After the People Have Spoken
If a candidate dies between the popular vote and the meeting of the Electoral College the parties follow the same process to fill the vacancy on the ticket. If the candidate that dies is on the winning ticket, it's still the party's responsibility to provide a new candidate their electors could vote into office.
But here, the political implications are more serious because it takes some of the power away from the people; they don't get to vote again. The replacement candidate's name goes on the Electoral College ballot only, and their political party expects its electors to vote the replacement candidate into office.
There's no federal law saying the electors have to vote for the new candidate. Theoretically, if the candidate to whom they pledged their votes dies and their party doesn't name a preferred successor, electors could vote for the party's VP candidate, a third-party candidate or a leading contender within their own party. But state laws vary on the matter.