Question

how saturated air turns into unsaturated air?

Answer 1

saturated air will become an unsaturated air if you raise the temperature alone(note that volume of air does not change).Similarly,if you decrease the temperature alone of an unsaturated air, it will become saturated.

The saturated condition can be expressed in terms of the pressure exerted by the water vapour.Water vapour,being a gas exerts its own pressure and thereby contributes to the total pressure of air.