No problem :)
The exact page number is going to depend on which version of the text that you are reading. It's from part 1, and in chapter 7. The quote is 5 paragraphs from the end of the chapter.
The full quote goes like this:
"The past not only changed, but changed continuously. What most afflicted him with the sense of nightmare was that he had never clearly understood why the huge imposture was undertaken. The immediate advantages of falsifying the past were obvious, but the ultimate motive was mysterious. He took up his pen again and wrote:
I understand HOW: I do not understand WHY."
The reason that Winston understands how the party changes the past is because it is his job. Winston works at the Ministry of Truth, and his job is to change historical records to whatever the party presently needs them to say. Winston absolutely understands how the changes occur. He does them. He just doesn't understand why. He doesn't see a point to it, especially when the historical events are old and obscure events that people barely remember any longer. Winston will eventually learn why the Party wants the past to say whatever currently benefits them. In the Party's own words:
'"Who controls the past controls the future: who controls the present controls the past,"' repeated Winston obediently.