How can we change position of fetus at 39wks from posterior to normal position for vaginal birth. He is 3.2kg and very active.

Not necessary. A baby's starting position for labor is not really important. The steps in childbirth are descent into the pelvis, internal rotation, extension, external rotation and expulsion. Long story short, the baby rotates as it comes through the pelvis, so starting out posterior is not a problem.