Nothing. The President is elected by the Electors, and Joe Biden received a majority of the votes of the Electors. There is no statutory or Constitutional basis or means to reverse the vote of the Electors.
Of course, if it appeared that Biden and Harris criminally procured, rather than merely benefited from, a stolen vote to choose the wrong Electors, they could be impeached and removed - but that would simply result in Nancy Pelosi becoming President, not Trump resuming office.
You think a Dem congress would impeach their own President? This isn’t the Republican Party we are talking about.