How can air launched UAV's be decelerated?
......the same way you decelerate any other kind of aircraft.  Once released from the Mothership at idle power, it slows down until reaching an appropriate cruise speed thence power is increased to maintain speed.

If the release speed of too high for the UAV, one might use some kind of a frangible capsule equipped with a drag chute or other high drag device of some sort to enclose the drone during launch, thence the capsule fractures once the speed is slow enough, releasing the drone to begin flight under its own power.