"Now, it’s still purely speculation, but should a presidential candidate drop out, the Democratic National Committee has rules in place to handle the situation. Article 2, Section 7 of the DNC Bylaws says that if there is a vacancy on the national ticket, a special meeting of the Committee ” shall be held on the call of the Chairperson,” where they would choose a new candidate. Such meetings make decisions based on a majority of those in attendance.
But what about if she has to withdraw after the election takes place, but before she is sworn in? Well, if a candidate withdraws after the general election, but before the electoral college meets, federal law says that electors can vote for whomever they want, although states can pass their own laws on the matter.
However, should a candidate win the election, but become incapacitated prior to the inauguration, then Section 3 of the 20th Amendment kicks in, according to the Office of the Federal Register. The 20th Amendment says that in such a scenario, the Vice President-elect would become President."