The word for "heaven" or "heavens" in Genesis 1 is "shamayim". Since the "im" at the end of the word is a plural marker, much like "s" in English, one understands why the translators of versions other than the KJV have translated the word as "heavens".
This is an account of the creation of the earth and, peripherally, the rest of the universe. It is useful to use the term "heavens" meaning both the sky and that which is beyond the sky in terms of the natural creation so as to differentiate it from the term "heaven" which is the dwelling place of God.
However, note that in 1 Kings 8:30 "heaven", the dwelling place of God, is a translation of the same word as used in Genesis 1. See also Deuteronomy 10:14; 1 Kings 8:27 where God's dwelling place is referred to as "the heaven of heavens" or ""shamayim" of "shamayim". Translating "shamayim" as singular or plural seems to be a judgement call on the part of the translators.
I hope you have found this helpful.