So let’s say the EC meets and votes on Dec14 and Biden is selected as POTUS. Come Jan 20 he is sworn into office. Shortly thereafter, positive overwhelming indisputable evidence comes out of Democratic cheating and “stealing “ the election and Trump was the rightful winner of the election. Will Biden be removed from office and Trump reinstated as POTUS? I kinda wish this would happen as I would love to see what will happen should such a thing occur.
There have been stolen elections ever since there was a United States of America. I think once the election is formally judged finished, and the winner is in office, it's over. Period.
Afterward, if fraud is found that reverses the election, the individuals caught can be prosecuted and put in prison to discourage that in future elections. But, there will likely be no way to force Biden out unless he or his campaign are proven to have conspired or participated in the fraud, it becomes an issue of punishing the candidate for the acts of others. Of course, a man of integrity who finds out the election was invalid would resign. The problem there is the VP takes over or resigns. Then the Speaker of the House. Basically, there's no Constitutional provision to undo an election result and inaugurate the rightful winner. Perhaps the Supreme Court could declare the electoral process invalid due to state fraud, but I'm not sure that can be done. If it could be, then they could order the electoral college to meet a second time with the valid electors there.
Either way, Trump could sue the corrupt states for costing him the election. At that point, he might be able to demand reimbursement of his personal funds spent on the campaign and the money spent by donors. If a casino has irregularities during a game, they cancel the bets and give all the players their money back. States that rig an election should be held liable for the money wasted on a no-chance-to-win election.
From a legal perspective, the states legislatures have the duty and power to select their electors. If a state disenfranchises its voters by committing fraud, it doesn't really affect the electoral college. It affects who the electors cast their votes for, but in the electoral process, the state did its thing -- they decided who gets their votes, and they gave their votes to that person. So, Constitutionally and Federal-law-wise, what happened at the state level is irrelevant. The electoral votes stand.
Now, if a state's voters were defrauded, they have the ability to throw those officials out, the state (AG) can file charges, or the FBI can come in if the election didn't follow the state's election laws. That fixes the next election, but the damage to the recent election is done and will remain.
Once Trump concedes, he's done. Best thing is for him to contest the results as long as possible.
There are remedies at the state and local level that may include voters voting the office holder out of office in a recall election. There's no such recall provision for President.