Answer:
By rewriting the archives of the London Times he makes them consistent with current Ingsoc policy.
Step-by-step explanation:
One of the most important features of the Ingsoc party is that in order to stay in power, they try to alter reality. This is done with the objective of manipulating people into believing their propaganda and therefore, remaining easy to manipulate. One of the ways in which they do so is by changing past editions of the London Times in order to make them consistent with current policy. This is the job that Winston performs, and therefore, it is absolutely essential to the survival of the party.