Question

Explain how if we change body's mass distribution , centre of mass will be at the same position if no external force acts on body.

Answer 1

If a system experiences no external force, the center of mass of the system will remain at rest, or will move at constant velocity if it is already moving i.e.the center of mass momentum is conserved. ​
In absence of any external force, the center of mass of a two body system will have zero acceleration.
Since, F = ma => 0 => a = 0
So even if we change the mass distribution, the center of mass of the system will continue its state of rest or of uniform motion.