https://en.wikipedia.org/wiki/President-elect_of_the_United_States

Scholars have noted that the national committees of the Democratic and Republican parties have adopted rules for selecting replacement candidates in the event of a nominee's death, either before or after the general election. If the apparent winner of the general election dies before the Electoral College votes in December the electors would likely be expected to endorse whatever new nominee their national party selects as a replacement. The rules of both major parties stipulate that if the apparent winner dies under such circumstances and his or her running mate is still able to assume the presidency, then the running mate is to become the President-elect with the electors being directed to vote for the former Vice Presidential nominee for President. The party's National Committee, in consultation with the new President-elect, would then select a replacement to receive the erstwhile Vice Presidential nominee's electoral votes for Vice President.

TL;DR: VP gets the nom