When it is shown that massive vote fraud in fact occurred and that Donald Trump in fact won by a landslide, the honorable thing would be for Joe Biden and Kamala Harris to resign their ill-gotten positions, leaving office with at least a little bit of dignity.
The Constitution says that in the absence of either, that the Speaker of the House would become president.
However, as the Democrats the past several decades have shredded the Constitution into meaninglessness, and as The People would never accept Nancy Pelosi as president, I think what will happen is that some moderate neutral person such as Senator Josh Hawley of Missouri would be named president, to serve until January 20, 2025.
And then Donald Trump would be free to run, and win, in 2024, if he chooses.