They can’t at this point, as the primaries have already happened. Their only option to change candidate now is to have the party select someone else.
I went digging through the list of other presidents that decided not to do a second term in the past, and the latest an incumbent president ever announced that they weren’t doing another term was towards the end of March in the election year, which permitted time for the primaries.
That also allowed for a lot more time for the new candidate to start putting together a campaign. It’s still on a time crunch relative to the normal situation, but they had seven months before the general election to sell themselves to the American public. We’re down to three-and-a-half months and we don’t even know if this is gonna happen.
EDIT: There is one perk of it being so late, which is that Trump has already selected his running mate. That means that unless Trump’s willing to dump Vance, it’s possible to build a ticket specifically optimized to beat Trump/Vance – Democratic strategists know what they’re facing in building their ticket, whereas Republican strategists did not.