the offical word is rectify but what word would winston use instead

He would use forge or change.

alter (alteration)

From the text:

"The messages he had received referred to articles or news items which for one reason or another it was thought necessary to alter, or, as the official phrase had it, to rectify."

Sure but if you were eating gruel with Winston one day at the canteen, he might also use forge or change as well as alter. He'd say it really quietly though so the telescreens couldn't hear!