George Orwell stated that "He who controls the past controls the future." How does this quote relate to Napoleon and his actions?

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.