As per my knowledge the centre of mass of a system can only be changed by a net external force acting on the body.

Now imagine that we are in a spaceship (obviously in a pressurized cabin with zero gravity). By the videos that I have seen of astronauts in international space station and other space related videos or movies I know that we can control our body parts movement such as moving our hands and legs while floating in space, without any external aid. This movement of our body should change our centre of mass but How is that possible ?