It would be helpful if you elaborated on where this plywood is located
If it's in an continued outdoor application and not already sealed... you're out of luck and need to start over if it's already buckled.
Most plywood for outdoors (unless it's expensive marine plywood is used for sheathing or if it's to be protected by something on top of it or above it that keeps water/moisture away...
Painting may help with lots of preparation and the plywood product you're using has not yet been exposed.
Plywood and in fact any wood will warp when it gets wet. It expands because the moisture being wicked up by the fibers within the wood. With plywood there are several layers of fibers in which to absorb moisture. You will find that the wood warps away from the wet side. As one side stays drier then the other it curves.
If the plywood is really thin it can be steamed slightly then put into the shape you want to maintain, then clamped in that shape. Wait a few days for it to dry and it should remain in that fashion. However it is really not practical or cost effective to do this.
Sealing both sides of any wood product is always suggested to prevent warping and twisting.