Once your piercing is fully healed: wash your hands, hold the bottom of the barbell, and unscrew the top ball (righty-tighty, lefty-loosey). Slide the old bar out from the top, insert the new one from below, and screw the ball back on until snug. Never force it — and if it's a dangle style, the charm hangs from the bottom ball, so the swap works exactly the same way.