How do you loosen the balls on a barbell on a nipple piercing?

You unscrew them once the piercing is healed enough to withstand the bit of pressure needed.

as it was said be sure to let it heal sufficiently. Make sure your hand are clean and dry, and that you're in an area with good light to see in case the ball falls. Grip it firmly between two fingers and twist - it might take a while depending on how tight the piercer put it on.