Leak in bladder. Unlace ball, remove bladder, and patch like a tire. See
tire.
Leak in laceless leather ball. Send it back to the factory.
Leather dried out. Inflate ball. Rub a damp cloth in saddle soap and rub soap vigorously into leather. Remove excess soap with a cloth dampened in water.
Seams torn. If the ball is laced, you may be able to get inside with your fingers and resew seams with strong cotton thread. A curved needle will help. But don't count on success. If seams in a laceless ball are torn, send the ball back to the manufacturer.