The party changed history because they believed
''If you control the past, you control the present''
The point is, people trust what they read in their books, what they are taught by their teachers and usually what their government tells them.
Hence, the main character of the book, Winston Smith is hired past historical records which are now deemed incorrect.
If the schoolbook history is changed, the present minds of the generation can be gradually altered to give them a new understanding of reality as they know it.
