Section 3 of the 20th Amendment to the Constitution of the United States begins with this:
If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President.
I am presuming at this point that no one is "the President elect" in the eyes of the Constitution until the electoral college votes in December. What would happen if Biden dies between now and the convening of the electoral college? If no states had laws against faithless electors, I would presume the electors could then freely decide. But there are such laws.