Why do we have 2 translations? Is the original Hebrew He shall bruise thy head correct, or the King James version It shall bruise thy head?
The word in question here in Gen 3:15 is: ה֚וּא (hū) which is: Pronoun - third person masculine singular. When referring to a person rather than a neuter object, it comes in two forms: הוּא masculine, and הִיא feminine.
Thus, the text of Gen 3:15 should be translated with the English, "he". This is consistent with Gal 3:16 and makes Gen 3:15 a messianic prophecy.
The LXX also has αὐτος = "he".
Cambridge Bible for Schools and Colleges makes an interesting observation here:
The Vulgate: "ipsa conteret caput tuum" is noticeable. By an error, it rendered the Heb. masc. pronoun (“he” = LXX αὐτός) by the feminine pronoun “ipsa,” ascribing to the woman herself, not to her seed, the crushing of the serpent’s head. The feminine pronoun has given rise to some singular instances of exegesis in honour of the Blessed Virgin Mary.
No two languages have have 1:1 correlation. When we translate, we often have to drop words or add them in the target language in order to convey precisely what it is that was conveyed by the source language (for example, the Hebrew "Behold me" in response to being summoned for becomes, "Here I am." or "Yes?"). A good translator retains as much of the meaning, rather than merely the form, as possible of the sentence (since retaining form can hinder the understanding of what was meant in the original, thus constituting a failure to actually translate).
So the answer is that both are correct; or rather, both are valid (there is no such thing as "the correct" translation of any given language into another, only valid or suitable, and at least grammatically viable ones).
It seems that the King James translators wanted us to view the "seed" or "offspring" as such, rather than an individual descendant or a set of descendants specifically.
And the Hebrew allows for this, because the pronoun hu here, while properly speaking masculine (being paired with the masculine noun zera: "seed") refers only to the masculine noun insofar as grammatically it is masculine, and not that the "seed" itself is masculine or one or many specifically.
The force of the Hebrew is "and the same shall crush..." without being specific. Thus "he" or "it" or even "they" (cf. Romans 16:20; Revelation 12:1, 11, 17) are all valid translations.