Not “heaven” as in a place where angels and God dwell, “the heavens” as in the skies and space. This describes the nature of the sky. It does not declare that God lives above us and can be reached with a stepladder. See commentary on verse 1.
Targum Onkelos: And the Lord called the expanse the Heavens. And it was evening, and it was morning, the Second Day.
Rashi: And God called the expanse heaven: Heb. שָׁמַיִם [This is a combination of the words מַיִם שָׂא, bear water (Gen. Rabbah 4:7); שָׁם מַיִם, there is water; אֵשׁ וּמַיִם, fire and water. He mingled them with one another and made the heavens from them (Chag. 12a).
Rambam: God differentiated the upper spheres from the lower spheres of the expanse, giving the whole the name “heaven.” The diurnal sphere revolved once; a second day.
Ibn Ezra: God named the heaven together with the air “sky.” Although God had not completed God’s work, the diurnal sphere revolved once, day blended into evening and night blended into dawn, a second day.
Diurnal Sphere: See commentary on verse 5