To strip off the existing finish and stain will require using a fiberglass safe paint/varnish stripper. Check out your local auto paint store for a suitable stripper. This is similar to stripping wood except no sanding can take place, sanding will destroy the fiberglass. After the stain/finish is removed the door can be refinished.
I do recommend having this done professionally, if possible. The cost will be around $400-600 depending on the design of the door and if there are also sidelights.