Presidential Death During the Election Process | Teachinghistory.org

How can a candidate for President be replaced if they die or are disabled?
There's not a lot of guidance here. The Republican National Committee have provisions for replacing a candidate for President who fails to reach election day due to death or disability,

RULE NO. 9 Filling Vacancies in Nominations

(a) The Republican National Committee is hereby authorized and empowered to fill any and all vacancies which may occur by reason of death, declination, or otherwise of the Republican candidate for President of the United States or the Republican candidate for Vice President of the United States, as nominated by the national convention, or the Republican National Committee may reconvene the national convention for the purpose of filling any such vacancies.

(b) In voting under this rule, the Republican National Committee members representing any state shall be entitled to cast the same number of votes as said state was entitled to cast at the national convention.

(c) In the event that the members of the Republican National Committee from any state shall not be in agreement in the casting of votes hereunder, the votes of such state shall be divided equally, including fractional votes, among the members of the Republican National Committee present or voting by proxy.

(d) No candidate shall be chosen to fill any such vacancy except upon receiving a majority of the votes entitled to be cast in the election.

I haven't yet found if the Democrats have prepared for this eventuality (and, according to the Blaze, they have responded to requests for information).