You will need to remove the loose stain. To do this use a course scrubby sponge and some warm water. This should remove any loose stain without damaging the door.

Now you have to get some gel stain with the same color. You also try a wiping stain but this type of stain is too thin to cover the existing stain. Apply the stain with a soft white natural bristle brush. This will take a little practice to get the right look. Apply the stain the the entire door and let dry.

After the stain has dried you will need to seal the stain with a varnish.