reinstatement

DT is being widely mocked for his declarations that the presidential election will be overturned and he will be reinstated as president.

And rightly is he being mocked, for his only evidence for this scenario is the Argument from Personal Incredulity, the incredulity being DT's refusal to believe that he could possibly lose; therefore arguments must be invented that the ballots were somehow miscounted.

But I have also seen mockery to the effect that overturning an election after it's certified is unknown to the US Constitution, ridiculing the whole idea that it could ever happen.

But that's not true. It hasn't come up in a presidential election, and the insertion of the Electoral College into the process may change the way the rules work, but it's a long-established process in congressional elections.

First, though, let's eliminate the term "reinstatement." This is about a challenge to the 2020 election, and who was the incumbent has nothing to do with it.

Challenges to Congressional elections were a common feature of the 19th century, and they were often successful. They lingered on into the 20th. This link will (I think) enable you to open a PDF with a CRS report on all contested congressional election cases 1933-2009. As far as I can tell from this, the last time a seated congressperson was ejected in favor of a challenger was in 1938, but it has been known to happen, usually (at least in more recent cases) because of vote-counting questions in exceedingly close elections.

Again, the Electoral College complicates this in regard to presidential elections. In some states, the electors are legally required to elect the winner of the popular vote; in others, it's only customary. Can the electors be overturned if they weren't legally required to abide by an overturned vote? On the other hand, if they were, can the other party's electors be summoned to cast a new vote? This has never happened, so it's murky, whereas with congressional elections the mandate is clear: Article 1 Section 5, "Each House shall be the Judge of the Elections, Returns and Qualifications of its own Members."

So it'd be unlikely that DT could overturn the election even if he had any evidence to do so, but inherently risible? No, the idea is not.