What is Winston’s job in the novel 1984 and how does it relate to party’s actions in regard to language and control?

Winston works as a clerk in the Records Department of the Ministry of Truth.
It's not enough that the Party controls the present and the future; it must also control the past. Otherwise the past and the memories they invoke could inspire people to challenge the official version of the truth put out by the state, with potentially destabilizing consequences for the regime.
Winston plays a part in this systematic distortion of the past by revising old newspaper articles and historical documents to keep them in line with changing Party policy. His job literally involves rewriting history. This includes doctoring photographs by removing "unpersons," those deemed enemies of the state. All historical artifacts that no longer serve any useful purpose are then thrown by Winston down garbage chutes called "memory holes."
The Party doesn't just control the past; it controls the language used to talk about the past. Hence the invention of the dehumanizing neologism "unperson" to describe those regarded as traitors. Such language is itself used as an instrument of control, a means of constructing an alternative reality in which whatever the Party says is true:

It was as though some huge force were pressing down upon you—something that penetrated inside your skull, battering against your brain, frightening you out of your beliefs, persuading you, almost, to deny the evidence of your senses.

Winston's senses tell him that the newspaper reports, photographs, and various other historical documents that he alters as part of his job are true. But the Party tells him otherwise. As Winston goes on to say, if the Party says that 2+2=5, then it does, whatever his reason might say to the contrary.
Winston realizes all too well that the past, of its very nature, can only really exist in the mind. Therefore, if the Party wants to control the past it has to control people's minds as well:

If both the past and the external world exist only in the mind, and if the mind itself is controllable . . . what then?

That's not to say that the past, any more than the external world, is some kind of delusion. Quite the opposite, in fact. It's just that it has a much more tenuous existence than the present, making it more vulnerable to the kind of control exerted by the state, in which Winston is such an unwilling participant.