No one has gone up to heaven except the one who has come down from heaven, the Son of Man. And just as Moses lifted up the serpent in the desert, so must the Son of Man be lifted up, so that everyone who believes in him may have eternal life." For God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life. For God did not send his Son into the world to condemn the world, but that the world might be saved through him. (Jn 3:13-17)
Jesus has come down from heaven. Since he has come down from heaven he would have to go up. Coming down and going up have significance in John’s Gospel. They have theological purposes. Coming down was not to condemn the world. It was to save the world. God sent his son into the world to because he loved the world. Go so loved the world that he was ready even to sacrifice his son.
Jesus will go up to heaven. He will go up because he has come down. It is all because he is the son of God. As son of God he has his place in heaven. His coming down and going up have reference only with regard to heaven, the place of God. His sojourn on the earth is time bound and motivated by a purpose. Purpose is to save the world.
Between this coming down and going up (while he is on this earth) there is lifting up (on the cross). This is the purpose and the mission for which he has come down. This lifting up links both his coming down and going up. If the mission is not completed (being lifted up) then his coming down will have no purpose and there is no reason for his going up. Salvation of the people is the only purpose.