Typically, when lumber is warped it is discarded or sent back to the lumber yard. But if you have no choice and need to use it, you can reverse the warp. Warped lumber is caused from moisture evaporating from the wood at different rates. Tension is created on one side, pulling the lumber into a curve. If you recreate the tension, let the lumber return to it's original shape and wait a few days you can probably use the wood.
Lay the warped timber on its edge on a concrete floor in a cool, dry place out of the weather.
Place one block on each end of the lumber.
Place another piece of scrap lumber parallel to the warped piece, sandwiching the blocks between the two pieces of lumber. The crown in the warped piece of lumber should be facing away from the scrap lumber.
Place a bar clamp over both pieces and tighten it to squeeze both pieces of lumber together. Tighten the clamp until you have a drastic reverse bow on the warped piece. Let the lumber sit for 72 hours.
Remove the clamp and check the lumber. If still not straight, put the clamp back on and tighten. Let it sit for 72 hours and then check every 24 hours and repeat until it is straight enough to use.
Try to find two pieces that are warped and use the warps against each other. Put both warps opposite each other and clamp.
If your lumber won't straighten, cut it into smaller pieces and utilise it any way you can. Some lumber will not flatten.