Found this, when I did a search.....
If a candidate dies before the general election but after they’ve secured their party’s nomination, it’s a relatively simple fix: The deceased candidate’s party picks a replacement (who may or may not be the vice presidential candidate from the ticket), and that replacement is on the ballot on Election Day.
“before the general election”
And since voting has already started then we are longer before the general election. Biden cannot be replaced in that manor.
What about those who voted during open voting. Are those votes counted towards the new candidate who was not on the ballot prior to Election Day?
“its a relatively simple fix ... and that replacement is on the ballot on Election Day.”
ballots are printed weeks in advance, so what happens when there’s not time to reprint them?