I see no reason to believe the debate(s) will not go exactly like they went in 2020:
The first debate will have a reasonable Biden, with an insane lying trump ranting about how he got reductions in all prescription drug prices by "80 to 90 percent", and how insulin was now "cheaper than water." Less than 30% of the voters will think trump won the debate. That means many of his supporters will even realize he is a loser.
trump will realize he is way over his head, and refuse the second debate. trump will hide like a scared child.
Unlike in 2020, there is no third debate scheduled, so that is a divergence from 2020. That means there will only be one debate.
There is a real chance that trump will do far worse than this, but no realistic chance that he will do better.
If the debates are the only thing that changes between now and the election (unlikely, but hypothetically speaking), then Biden will win the election. trumpers point to polls where trump is getting 45% of the vote, and Biden is getting 40%, and say that trump is leading. trump getting the last 5% of the vote necessary to win would be difficult in the best of times. The 15% who are undecided do not like trump. Many have decided they will not vote for trump, but have not decided to vote for Biden. A terrible debate performance would mean that the undecideds and a few of the voters who were willing to vote for trump will go for Biden.
trump desperately needs something very strange to happen to turn this freight train around.