Jan 6 is the important date, because that's when Congres counts the EC votes. That's when the next holder of the office is formally confirmed. If a President-Elect dies after that , there's nothing that can really be done about it. I would assume the Vice President-elect just cuts to the chase and takes over as President on the 20th, but it's never happened before, so who knows?

If it happens sufficiently before Jan 6, there may be time for states to direct electors to vote differently. But that might be perilous: an EC majority is required for each office. If some states leave the ticket untouched while others make changes, you run the risk of splitting the vote so nobody technically "wins". I don't think any states would direct electors to vote differently unless they all do. And it's hard enough to coordinate a single group of politicians to do anything, it would be nearly impossible to get 30+ states to all pass the same legislation at the same time.

But could Congress realistically allow EC votes to be counted for someone who can no longer take the office? There will be a big push to invalidate those votes, particularly given the GOP's tendency to piss on elections these days. I would not put it past them to use their power in the House to deny Democrats the Presidency even if Biden plainly won the EC , but bites it before Jan 6.

To make matters worse, the VP presides over the vote counting as President of the Senate, but the VP office would be vacant because whats-her-name would end up being President. They would need a vote in both houses to name a new VP, and you know the GOP Majority in the House would just sit on it and not act to confirm the new VP. So it might be the President ProTem of the Senate, and Democrats have a real chance of losing the Senate this year.