There is no Classical Greek word for 'it' but we can hardly avoid the word in translation. W.D Ross's rendering makes clear that the mythical 'it' refers to virtue (arete). We have seen, Aristotle says :
That virtue, then, is concerned with pleasures and pains, and that by the acts from which it [virtue] arises it [virtue] is both increased and, if they [the acts] are done differently, destroyed ... [W.D. Ross, The Works of Aristotle, IX. ed. J.A. Smith & W.D. Ross, Oxford : OUP, 1949 reprint.]
I don't think anybody could claim that this is a very clear passage in the Gk or in translation. We ought perhaps to recall that no small part of Aristotle's text, here and elsewhere, consists of lecture notes rather than polished drafts. I can't myself see the logical consecutiveness of 'virtue ... is concerned with pleasures and pains' and 'by the acts from which it [virtue] arises it [virtue] is both increased and, if they [the acts] are done differently, destroyed'.
But we should note that the passage ought not be read, analysed and interpreted as a fragment isolated from its context. It needs to be taken against the background of NE II.3 as a whole. In which case ...
The general idea appears to be that virtue is built up as a hexis or habit by the doing of actions. According as we act in certain ways, we will acquire virtue and experience pleasure in exercising it. But if we act in other ways, guided by the pursuit of the wrong pleasures or of the right pleasures in undue measure, we will be deflected from virtue by pleasure and lose whatever virtue we had or never fulfil our potential for virtue in the first place.
I offer this reading tentatively. Commentators, I note, generally omit discussion of this particular passage. There is a more sophisticated and informative discussion of Pleasure in Bk X.