The steps involved with stripping a wood stain can be extensive depending on the type of stain and if a clear finish, like varnish, was applied. Follow the steps outlined here and skip any that don't apply.
Remove the door and protect any areas you don't want damaged during the refinishing process. It's easier to work on the door if it is on saw horses.
Use a thick, gelled, paint stripper to remove any varnish or clear finish . This will also remove some stain. Follow the manufactures instructions for rinsing and drying of the wood.
If the wood isn't lightened enough for your new stain, purchase and apply a wood brightener. The 2 part hydrogen peroxide types work the best and will remove a lot of the remaining stain. This is powerful stuff, so be careful and follow the instructions.
After all the wood stripping and allowing the wood to dry, sand all door surfaces smooth. Using chemicals and water for rinsing will raise the grain. Start with 150 grit then finish with 180.
99% of the wood stain should be gone, but if any remains sanding is the only option left to get rid of it. Most likely the new stain will cover any remain color. It is best to sample any wood stain before its application. Use a piece of wood that is the same species and approximates the grain and color.
Now you can apply the wood stain. Gel stains provide more control of the color and intensity. Most types of wood stains will require a clear finish for protection.