It’s over. At that point, the outcome can’t be changed. New electors can’t be appointed in any state, by legislatures or any other means. No time machine exists to undo the meetings of each state’s electors that already have occurred.
There is nothing for Congress to do except to accept that Biden has won based on a majority of the electoral college ballots cast on Monday.
Of course, Congress still must receive and count these electoral college votes and formally pronounce Biden the winner, in a special joint session on Jan. 6. But that will be a mere formality. No officially sanctioned slates of rival electors — from state legislatures, as previously feared (and urged by President Trump), for example — exist for Congress to decide between. Republican senators can go ahead now and publicly acknowledge the result.
Indeed, the delay between the electoral college vote and congressional announcement of that result is a relic of the 19th century, with its slower methods of transportation and communication. After the electoral college has elected Biden president, there is no reason — other than sheer obstinacy or reality-denialism — to withhold public recognition of Biden’s victory.