Wood can be unwarped by using weights to straighten it out. Wood warps when moisture gets trapped in the wood and it then dries out.
To straighten a wooden door, the door must be unhinged, or de-attached, and then placed on a surface where the warped ends can hang in the air. Weights must then be placed on the ends of the door to apply constant pressure to bend the wood back into its original shape. Getting the wood to bend back into its original shape can take some time: usually a few days or even a few weeks. After the wood has been bent back into place, it may need to be re-sanded or refurbished to deal with any new marks caused by the warping. After sanding the wood down, an oil-based primer should be applied so that it can then be repainted. If the wood is taking too long to bend, wet cloths can be placed on the door. This will speed up the bending because the added moisture will give more flexibility to the wood. The technique of using weights to reshape warped wood can be used with any wood that needs to be straightened out.