First, use a chemical stripper to remove the clear finish on the doors. Some wood stain might be removed as well. Rinse the doors with water and allow to dry.
Now that the "finish" is gone, you will have to decide which method to use for removing the remaining stain or lightening its color. The choices are Two-part Hydrogen Peroxide or using Oxalic Acid. Both are sold through your local paint store or furniture refinishing supplier. Carefully follow the instructions provide by the manufacturer.
Each method will lighten the wood, but getting the doors to a true natural appearance will require sanding. The sanding can be done after the bleaching when the doors have dried.