In the book, Napoleon does a lot of lying about the past. Because he lies about the past, you can say he is controlling it (because the animals only know his version of it -- they forget what really happened).
Think about some of the ways he changes the past. He makes everyone believe that Snowball was an agent of the enemy even though Snowball was really trying to help the animals. He keeps changing the commandments until they come to say something very different from what they used to say.
So he is controlling the past, making it look like things happened some other way than how they really did.
How does this help him? By making Snowball look like the bad guy, Napoleon makes himself look better and makes the animals be loyal to him. By changing the commandments he makes it look like Old Major would have approved of what he is doing. In both cases, that makes his power more secure.