The short answer, conservative election law expert Jim Bopp says, is no. Unless Trump himself drops out, which Trump says will not happen, the party is stuck with him.
The longer answer is that it would be difficult — but is possible, according to conservative election law expert Robert Kelner.
The key part is "Rule 9," which governs how the party fills "any and all vacancies" of Republican candidates for president or vice president:
The Republican National Committee is hereby authorized and empowered to fill any and all vacancies which may occur by reason of death, declination, or otherwise of the Republican candidate for President of the United States or the Republican candidate for Vice President of the United States, as nominated by the national convention, or the Republican National Committee may reconvene the national convention for the purpose of filling any such vacancies.
What Rule 9 really means is a source of debate.
According to Bopp, "It only authorizes the RNC to fill a vacancy if it occurs, i.e., for instance, if he steps down."
According to Kelner, "Rule 9, as currently framed, includes clear wiggle room. It includes a catch all for a vacancy created 'otherwise' than in a classic death or withdrawal scenario. The RNC rules are filled with such negotiated weasel words to allow flexibility to deal with unanticipated scenarios."
Bopp, though, says that the rule is more limited. Of the "otherwise" sentence, he explained, "This sentence only empowers the RNC to fill vacancies, not create them."
Bopp's main point, and the reason he believes the rule gives no power to the RNC to replace Trump is that, as he puts it, "The power to create a vacancy is a separate and independent power from the power to fill vacancies and that power would have to be conferred on the RNC by a specific rule, which does not exist."
Discussing Bopp's argument with BuzzFeed News, Kelner explained why he thinks that's wrong: "I somewhat agree. But he is missing the point. Nothing in the RNC rules PROHIBITS the RNC members from removing a nominee. For example, if a nominee committed a serious crime (I'm not suggesting that's the case here), surely he could be replaced by the party. Nothing in the rules prevents it, for good reason."
If the party decides that Trump "is no longer a viable candidate or that he has engaged in malfeasance," Kelner said, "the RNC members could vote to remove him." He added that, as to Trump formally being the party's candidate, that is "purely a matter of RNC rules, which the RNC itself interprets" — not a matter of federal or state law.
Even Kelner acknowledges, though, that replacing Trump as the party's formal candidate would not solve things for the party.
The "tougher issue" than formally replacing Trump, he said, would be "the revision of state ballots, which would likely be possible in some but not all states." As others have noted, voting already is underway is several key states, including Florida and North Carolina.
"It wouldn't be easy by any stretch," Kelner says. "But suggestions that it is not possible as a matter of law are simply wrong."