Common sense, rational thought, and the ability to take it step by step.
If you will look at the linked cnn story, you will see that it states that the Biden and Trump campaigns had agreed to the 2 debates. How did that happen, you might wonder. Trump had said he would debate Joe anytime, anywhere, several times. Widely reported. So when Biden challenged Trump to dates, times, network moderators, and the debate rules, Trump accepted the challenge to debate JOE BIDEN within just a couple hours. So both of those parties were then obligated to show up to debate each other, per the agreed terms. There were no amendments or "what if" provisions allowing for a substitute. Therefore, when Joe got his ass kicked in the first debate and then had his nuts cut of by Pelosi, Obama, Schumer, and the Dem Big Buck donors, he dropped out of the presidential race. Therefore, the original agreement became moot, null and void.
Trump then later proposed a debate to Harris on FNC with Brett Bair and Martha McCallum as moderators. Two FNC hosts that have done an excellent job previously, and with FNC leaning right it would provide a good balance from the first debate on left leaning CNN. (I will say that I thought the two CNN hosts did a much better job than I expected, being pretty fair about the topics and questions). But of course, Kamalalala is scared to death of getting actual serious questions from FNC, and is doing everything she can to avoid debating Trump, especially on Fox. You KNOW that is true since it has now been two weeks with her as the presumptive nominee of the Dem party and she hasn't done any press conferences or faced any tough questions yet. How long can she keep that up, insulated in her protective cocoon, at great distance from any media person that is libel to actually ask her a question of substance?