Where is heaven?

Acts 1:4-9 Promise of the Father

Acts 1:10-11 Heaven

Acts 1:12-15 Sabbath Day's Journey
ACTS 1:10-11  10 And while they looked steadfastly toward heaven as He went up, behold, two men stood by them in white apparel, 11 who also said, “Men of Galilee, why do you stand gazing up into heaven? This same Jesus, who was taken up from you into heaven, will so come in like manner as you saw Him go into heaven.”

Where is "heaven" (Acts 1:10, 11)?
There are three places that the Bible calls heaven. The first heaven is the earth's atmosphere, which God made to separate the waters on the earth's surface from the water vapor in the clouds above: "Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. And God called the firmament Heaven..." (Genesis 1:7-8). The second heaven is the outer space that lies beyond the earth's atmosphere and where God put the sun and the moon: "Then God made two great lights: the greater light to rule the day, and the lesser light to rule the night. He made the stars also. God set them in the firmament of the heavens to give light on the earth, and to rule over the day and over the night, and to divide the light from the darkness..." (Genesis 1:16-18). The "third heaven" (2 Corinthians 12:2) is the "habitation" of God from where He looks down on earth - "Look down from Your holy habitation, from heaven, and bless Your people Israel and the land which You have given us..." (Deuteronomy 26:15)

Into which of the three heavens did Jesus "go" (Acts 1:11)?
He traversed the first two and returned to the third, where He prepares for and welcomes Christians upon death on earth: "In My Father’s house are many mansions; if it were not so, I would have told you. I go to prepare a place for you." (John 14:2)

Who were the "two men" mentioned in Acts 1:10?
Angels. λευκη (leuke), the Greek word translated "white" in "white apparel," also means "radiant" or "brilliant."

How did Jesus come to earth the first time?
Quietly, like a camouflaged paratrooper on a mission landing behind the enemy lines (see Birth of Jesus)

How will Jesus "come" (Acts 1:11) to earth the second time?
Loudly, without the camouflage, as who He is - "For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God..." (1 Thessalonians 4:16) - and escorted by His entire army of angels: “When the Son of Man comes in His glory, and all the holy angels with Him, then He will sit on the throne of His glory." (Matthew 25:31)

What will Jesus do when He returns to earth?
See Son of Man.