Not an expert on the US political system, but legislative power, and thus changing the system, is vested in Congress, not the Presidency. So he’d need to get a supermajority there, and rewrite the Constitution. Point is, it’s not Trump that can do it, but a theoretical Republican House and Senate 60% supermajority (or a supermajority of State legislatures).
Look at what happened in Hungary in 2010, that’s a blueprint of that happening. Biden dropping out is good news, since from the outside it kinda looked like the US might have been setting itself up for that kind of thing to happen. I think with Harris running, it’s not realistic the Dems will lose that bad.
That said, he could try another Jan 6th, basically take Washington under military occupation. Then it’s up to which states go along with it and which ones don’t, and you’ve got a civil war.