I’m interpreting this as the one large or small change that would most improve each film in a vacuum.
TPM: Tweak Jar Jar’s character so that he is a jaded observer of the galaxy and a cynic of everyone in it.
AOTC: Anakin and Padme go to Tatooine out of a shared concern for Shmi due to Anakin and Padme having worked together to free her and many of Tatooine’s slaves some years prior, only to find that she is dead by the time they get there. Anakin’s murder of Tuskens is re-contextualized as a dream.
ROTS: After Anakin saves Palpatine from Mace Windu, Palpatine immediately sends Anakin on the mission to defeat the Separatists on Mustafar. As Anakin kills the Separatists Palpatine initiates Order 66, and the trauma of the massive death toll sends Anakin over the edge. He realizes the situation is out of his control but his final choice isn’t made until Obi-wan arrives to confront him, assuming the worst.
ANH: Instead of a speeder, Luke takes his new, souped-up Skyhopper out to search for R2. He reluctantly parts with the beautiful craft in Mos Eisley, which makes his disgust at Han’s ugly ship all the more dramatic.
ESB: Give Han and Leia a shared experience they discuss during the film where they expressed their attraction and even had a relationship until Han got cold feet regarding the Rebellion.
ROTJ: Leia is not confirmed to be Luke’s sister. ‘The other’ Yoda mentioned in ESB is explained to be a vision where he sensed another Skywalker defeating the Emperor, and Yoda postulates that Luke could have a twin. Obi-wan says that this is possible, and Luke suspects that it is Leia which Obi-wan neither confirms nor denies. The vision is fulfilled as Anakin destroys the Emperor and it is left unclear what Luke believes at the end.
TFA: The First Order is just the Imperial Remnant using their old battle-worn tech and the Death Star prototype seen in ROTS with the equatorial weapon. This weapon is still just a skeleton of a station which has only a hyperspace ring, but it is sent to crash into the Hosnian system. The Resistance dogfights through the structure in an attempt to find a central generator to destroy the station, but it’s so incomplete that the mission fails and the station crashes into the Hosnian system, destroying the capital.
TLJ: Canto Bight is located aboard the city-ship Supremacy. Finn and Rose infiltrate the ship and go to the onboard casino in an attempt to find out how Hyperspace tracking works from the scientist who developed the tech…which the First Order keeps near for security (Notes of Jango Fett, it rhymes). They find DJ instead and he betrays them, but Rose gets a good look at the tracking room and realizes that a part of the Supremacy is permanently in Hyperspace in order for the tracker to work. Finn gets the message out to Poe who relays it to Holdo, who then in full knowledge of this weakness uses the Raddus as a Hyperspace ram.
TROS: Get an actual writer.