The Democrats are creating a constitutional crisis by trying to impeach a citizen who is no longer the president. The constitution is clear, "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." How can you remove someone from office who is no longer in office?
Here's my solution. It does take a bit of an extraordinary effort but since the Democrats are so intent on pursuing the matter, I'm sure they will be fully supportive. Congress needs to convene and pass the Trump amendment. It would specifically, and for Trump only, remove Biden from the office of the President and reinstall Trump as the sitting President. The Democrats could then constitutionally pursue his removal from that office though impeachment.
Like I said, you're welcome.