Plywood pieces can warp if they have been improperly stored or exposed to moisture. The bowing that occurs is actually the plywood shrinking. You can flatten them again with hot water and a warm environment. This process will take several hours, but once it is finished, the plywood will be rejuvenated and ready to use for your project.
Spray hot water generously onto the concave area of the warped plywood. The concave area is the interior of the hump formed by the warping.
Set the plywood sheet on a flat surface that has adequate exposure to warm air, such as a paved driveway on a warm, sunny day. The convex, or humped, side should face up, and the side you sprayed with water should face down.
Set a heavy object or weight on top of the hump to help it flatten out quicker.
Allow the plywood to dry in the sun, checking its progress every five to 10 minutes. Once it is flat, place the plywood in a cooler spot, such as a basement or garage.