Unscrew one of the balls and simply pull your barbell out. If you are still healing, using a dab of triple antibiotic ointment on the piercing will help ease out the barbell, and make it easier to insert a new one.
Drawing from personal experience, I should mention that depending on how long you've had the piercing hole will determine how painful removing the hardware will be. There isn't much to know about removing a barbell except that after removal, simply disinfect the open hole with soap and water.
If you can't easily slip the barbell out, see your doctor asap. It should come out with ease, maybe use a little neosporin to lather it. Be sure to clean it well daily with Dial soap until it's fully healed, use Vitamin E to help with scarring.
If the piercing is not infected you should be able to slip it right out. If it is inflamed and sore cumsult a proffesional piercer. You culd infect it further if you try to remove it.