Thin wood boards, especially wide ones, are notorious for warping (cupping) especially if not stored properly. But this is preventable, and even very badly warped wood usually can be flattened with a bit of patience.
The boards warp when they release or take on more moisture on one side than the other. If one side of the board dries faster than the other, the fibers on that side contract causing the board to warp.
To get the board to flatten back out you will need to dampen the inside of the cup (the side where the fibers are dryer/tighter). This will cause the fibers to expand and the board will start to flatten. To do this we usually use a damp cloth and evenly wet the one side, you can also use a spray bottle. Leave the board lay wet side down on a workbench overnight. Repeat if it doesn't go completely flat or warps back the other way. Some weight on top to help flatten the board might be necessary for stubborn ones.
To store the boards to avoid warping in the first place, keep them stacked with thin sticks between each one so they can dry evenly on both sides. Or keep them all in one tight stack with weight on top. The goal is not to let one side get more or less air than the other. Once the board has acclimated to the climate in your shop it should be fine (this could take up to a week), unless, of course the humidity suddenly changes drastically.