If you prove fraud in a court of law, then you no longer have somebody who was chosen to be president using a constitutional path.
You have an installed president with an incomplete constitutional process, and an existing constitutional process that would need to be completed.
I have no expectations that will happen though.
Again, the Constitution makes no mention of individual citizens voting in a popular election for president. The votes were duly cast by Electors, counted, and recorded.
Remember when Trump wanted some state legislatures to appoint Electors that would vote for him rather than according to the popular vote count?
He did not succeed in this. The votes were cast by the Electors, counted, recorded, and Biden was inaugurated. Too late now. No court is going to entertain this, nor should it.
You are wrong.
The only thing the Constitution says about appointing Presidents is that the State Legislatures appoint Electors, who then vote in the only Presidential election that exists under the Constitution.
The appointed Electors did exactly that, on December 14, 2020. No state legislature, not Wisconsin, not Michigan, not Pennsylvania, Georgia, or Arizona disputed these appointments, which they had every right to do.
The election result was Biden 306, Trump 232, and Biden is the President while Trump is not.