Trump seemed quite defiant yesterday, so I don't see him quitting. I think it's more likely Pence drops out, trying to salvage his political career. Trump would probably try to replace him with one of the sycophants from his inner circle, like Giuliani or Christie.
By thew way, Giuliani is hitting all the Sunday morning talk shows, replacing Priebus and Conway on a couple. Conway has been conspicuously absent over the last 48 hours.
Regarding process, my understanding in the unlikely event of a Trump exit, the RNC would convene and choose a successor, which would almost certainly be Pence. That said, ballots are printed and voting has begun in some states, so it may be impossible to get Trump's name of the ballot in most places. Write-ins would almost certainly have to be the mechanism.
"It does me no injury for my neighbour to say there are 20 gods or no god. It neither picks my pocket, nor breaks my leg."
-- Thomas Jefferson