If Pennsylvania, Wisconsin and Arizona found fraud or malfeasance to the extent that Joe Biden's margins of victory in those states would be overcome, and the November outcome called into question, those states could decertify their Biden electors.
If they did, Joe Biden's electoral vote count (on paper) would drop to 265, leaving him short of the 270 needed to win.
Would Congress convene a special session and consider changing the electoral vote count?
It is impossible to tell at this point what, if any, mechanism exists to reverse the outcome of the presidential election.
"Let's see what happens."