Before I get to that, let's look at the concept of "willing suspension of disbelief". In order to enjoy any science fiction, fantasy or superhero movie, you have to accept the premise that time travel is possible, some people can fly or dragons exist, despite the evidence of physics and other real science. People who enjoy these genres are apt to not blink an eye at a teenager inventing "web slingers" and using his "spider sense" to fight crime, while scoffing at how quickly some toast pops out of the toaster. I'm one of those people. But even if one is willing to accept the impossible in order to enjoy the story, the non-real aspects must be internally consistent, in other words, the impossible physics, super powers et al must always be treated the same way. In Back to the Future II, the internal consistency is broken.
In the first movie, Marty, due to his presence in his past and his interaction with the teenage versions of his parents, was making changes that had immediate affects in the future (the "present" that he had traveled from). This is illustrated by the family picture that he carries: as his parents' romantic encounter becomes less and less likely, he and his siblings begin to fade from the picture, implying that changes in the past propagate forward in time without delay. When the timeline is repaired, Marty's picture is restored.When he returns to 1985 his present is changed in several ways as a result of changes in the manner in which his parents met.
In Back to the Future II this rule seems to have been skipped over. Marty & Doc (and Jennifer) travel to 2015 to save Marty & Jennifer's future kids from making a decision that will land them in jail. While there Biff steals the DeLorean, travels back to 1955, gives his teenage self a book that enables him to predict the results of fifty years of sporting events, and comes back to 2015, with Doc & Marty none the wiser...until they return to 1985. They find that Hill Valley is a dystopian society due to Biff's malign influence, a change made when Old Biff gave Young Biff the sports book. The change, like in the first movie, happens instantaneously, except for one detail: why didn't the change propagate forward to 2015? When Biff returns to 2015, it should have been a time 30 years after the nightmare Hill Valley, with him as top dog, head of a gambling empire, but nothing changed! Doc and Marty don't notice any changes either.
Some time travel fiction portrays the changes as so instantaneous that a person who is aware that time travel is occurring will actually see reality change around them. An example of this is the Netflix remake of Frequency. Changes to the timeline made by one character in a second character's past are shown as a quick blur, with everything subsequently different. One scene has a detective being led into the station, under arrest for an execution of a suspect; then time changes, and between one step and another she is not under arrest, but is being briefed on the case.
Another oddity, but internally consistent nonetheless, is how time travelers, when returning to their native time, are unchanged, even when everything else is. Marty is exactly the same, even though everyone around him has changed. In the changed timeline from Back to the Future he would have had a completely different upbringing than in the original timeline. In some time travel fiction the returning time traveler holds two sets of memories, pre- and post-change. The Butterfly Effect is a good example of this. In Back to the Future II it's a little more extreme since in the "Biff Timeline" Marty was in military school (or something like that) and Doc was in a mental institution. That's a tough one to make work, so I'll cut them some slack!
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