This question appears to assume the popular mythology that Satan is a fallen angel. In fact, the Bible contains no such teaching. (See “Is Satan a fallen angel?”)
There are two passages that talk about Satan “falling”.
He [Jesus] replied, “I saw Satan fall like lightning from heaven.”
Here, Jesus is responding to his disciples reporting how they had been able to cure people of illnesses. In the Bible, “falling from heaven” is figurative of losing authority. For example, Isaiah 14:12 describes the king of Babylon as “fallen from heaven” (see also Jeremiah 51:53), the king of Tyre was “thrown to the earth” in Ezekiel 28:17, and Israel’s splendor was hurled “from heaven to earth” in Lamentations 2:1. While the disciples were amazed at the power they had over disease, Jesus was thinking of his ultimate triumph over sinful human nature. Elsewhere Jesus emphasised the link between sin and illness (e.g., Matthew 9:5), and equated Satan with temptation to sin (Matthew 4:10). In other words, he is saying “You’ve seen illness cured, but wait until you see sin, the ultimate sickness, destroyed!”. Thus, the verse is parallel to Hebrews 2:14:
Since the children have flesh and blood, he [Jesus] too shared in their humanity so that by his death he might destroy him who holds the power of death—that is, the devil—
The second verse that mentions Satan falling is from Revelation.
The great dragon was hurled down—that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him.
Revelation is highly symbolic, and we have to be careful in identifying the symbols used. The dragon called Satan represents something, as do all the other features of this parable-vision such as the seven heads with crowns on, the ten horns, the tail catching hold of the stars, the angels and even heaven—all of these represent something. It makes no sense to try to understand this vision literally. Who ever heard of a fallen angel with seven heads and ten horns!
Also, look at the context. The very next verse (Revelation 12:10) says that as a consequence of the dragon’s downfall, people will rejoice because God’s kingdom has come. This shows that the event described in this vision must be still future, since we know that God’s kingdom has not yet come.
Most likely, the dragon being hurled down represents the defeat of the power of sin and the end of human kingdoms when Jesus returns to establish his kingdom. There are clear parallels with the fourth beast of Daniel 7 which also had ten horns. It represented a human kingdom that would be overthrown when Jesus returns (Daniel 7:19-23).