What are some instances in 1984 where the party has changed history?
The party says that they invented airplanes.
Winston's job is to change history so anytime he gets a correction to a news article it changes history. For instance, if they were at war with Eurasia but are now at war with Eastasia, Winston has to change it so that no one knows.
If the government got the rations of chocolate incorrect, Winston has to change it to make it look like the party is never wrong.