If a soccer ball is flat because it has been sitting unused and has lost pressure, reinflation is the answer. Pump the ball by hand or with an electric pump until it feels firm when pressed hard between the heels of both palms. If the ball is flat due to a puncture of its bladder, you can repair the ball in a process roughly similar to that employed for a flat bicycle tire.
Inflate the flat ball fully. Place it under water in a sink or bucket. Apply very light pressure on the ball until you see a stream of bubbles indicating where the ball was punctured.
Score a three-sided flap the size of a rectangular bicycle repair patch, centered on the puncture, in the vinyl cover and the underlying foam, using a utility knife. Do not press so hard that you cut the underlying bladder. Cut through the scored surface with a small sharp scissors.
Pry up the fabric underlying the foam and cut with the scissors on three sides to create a flap and provide a good view of the exterior of the bladder.
Apply rubber cement to the bladder. Allow it to dry for three minutes.
Peel the backing off the bicycle patch and apply over the puncture. Rub the patch down well.
Apply household glue to the surface of the bicycle patch. Push the flap back into place. Tape the flap into place with foot-long strips of duct tape stretched tightly around the ball horizontally and vertically.
Allow the patch to dry overnight. Remove the duct tape. Pump up the ball so the surface is hard again and it is playable.
Tips
Alternatively you can use an injectable patching sealant inserted through the valve. After injecting, bounce the ball 20 to 30 times. The material coats the inside of the bladder and seals the leak.