From what I have read, up until the Electoral College votes, the DNC can replace a candidate with whomever they want.

Edit - if the candidate dies. They can't do it at will.

Sauce - https://www.google.com/amp/s/fortune.com/2020/10/17/what-happens-if-trump-or-biden-dies-before-election-2020/amp/

There is a lot of legal gray area, so if you're going to disagree with this post, you'd better have something more substantial than your feelings to back it up.

From what I have read, up until the Electoral College votes, the DNC can replace a candidate with whomever they want.