Election date in fixed in the Constitution; it cannot be changed.
On replacing him, that's really uncharted waters as far as the presidential race goes. We're not only close to the election: voting has already begun. Tens of millions of absentee ballots have been delivered to voters, and I believe something around 1m have already been officially received. We're already past the deadline for candidates to be listed on the ballot in most/all states.
Per Rick Hasen (who knows this stuff), the most likely scenario if Trump dies is that the election goes ahead with Zombie Donald Trump on the ballot. It's too late to do anything else. This has happened in other races: former US Attorney General John Ashcroft infamously lost his Senate seat to a guy who'd been dead for three weeks. What happens after that point could theoretically get really weird, though:
https://electionlawblog.org/?p=116098
In short, it could potentially give the GOP an in to do something that Trump has mused about: using state legislatures to do an end-around on the way states award their Electoral College votes (which is usually by, y'know, giving them to the candidate who gets the most votes). They could argue that the presence of Zombie Trump means that the actual vote does not reflect the will of the people, and attempt to choose a slate of electors that would back Mike Pence. That's not incredibly likely, but it is a possibility if the election is close enough; it'd be the functional end of American democracy though, so I'd rather that not come to pass.
Everything is going to be messy now though. Biden was winning fairly comfortably, and the race has been the most stable (by polling) since polling became a regular feature of American political life. Now if Biden wins, there will be cries that he has done so only because Trump got the virus. I'd have much preferred that Trump fell ill the day after the election.
I’d be careful with assumption such as “Biden was winning fairly comfortably” - remember Hilary was on the run up to the last election. Then “something” happened ....
