I am assuming that I can't bring Carrie Fisher back ... as much as I'd like to and that we'll still be surprised by her sudden passing.
Film by Film:
The Force Awakens: Make the Republic more of a factor to better explain the political dynamics and to give Leia more presence. Have her arguing with a Republic big-wig about whether or not the First Order is a threat. Have Starkiller base take out the Republic Fleet that was conducting training maneuvers in the Hosnian System, not the Republic government. And rather than have the Resistance base being the target of Starkiller Base in the final battle, have them target Coruscant and the Republic Government. This change is to give more room between TFA and TLJ and to make the universe feel different, so TFA doesn't seem like such a ANH reboot.
The Last Jedi: No major changes. I don't want to get rid of Canto Blight, but I do think it needs some editing. I'd plant some more seeds about Finn possibly being Force sensitive here ... maybe have him trigger a switch unintentionally during their escape. I'd also have to, again, make the broader situation clearer, describe the Republic as "in chaos". Insert a line or two from Snoke that obliquely references Palpatine. Oh, and since the Resistance base was still hidden due to not knowing it's location in TFA, have it been discovered due to a First Order spy. That makes the Holdo and Poe conflict a little different because of paranoia in the Resistance's ranks. I'd introduce an officer that is siding with Poe turns out to be the spy.
The Rise of Skywalker: This is the movie that I want to redo the most. Rey is not a secret Palpatine. Cut the fetch quests. Don't introduce Palpatine in the crawl. Instead, start with a new scene, where we have the Republic Government debating how to take the fight back the First Order, only for them to be all blown up by a terrorist claiming to be working for Palpatine. This is probably where I'd kill Leia, obviously she was supposed to have been a more major player, but having her about to rally the Republic gives her a sudden, but more interesting death. Rather than have Kylo get to Palpatine in the opening scene, I'd make it a race, as both Rey and Kylo are trying to find the Emperor's hiding place.
I dunno ... I think TFA and TLJ can skate through pretty intact, I just want to remodel tRoS to feel less disjointed. Make Palpatine's survival not a part of the crawl, make it a mystery. Even whether or not he's alive, sure the terrorist said he was working for Palpatine, but maybe he was lying.