All loose and peeling stain/finish must be removed. This can be done by carefully using a brass wire brush and stiff putty knife. Do not damage the underlying fiberglass surface with scratches or gouges. Sanding isn't recommended as this can damage the underlying fiberglass, leaving smoother spots that will show when stained and finished.
Now wash the door with warm water and a little mild dish soap. A scrubbing sponge can be used. Alternate- use Paso Liquid Sander/Deglosser to clean and degloss the door. Be careful as this solvent could remove any exposed stain.
As long as only the finish was peeling, the stain is still intact, then restaining is easy. If there are spots of exposed fiberglass (light tan color) then these areas will need some stain first before restaining the entire door. Try to blend these areas into the surroundings and let dry. Now restain the entire door, let dry then apply a good clear finish.
Another option is to fully strip the door using a fiberglass safe paint stripper. This is more work and will take longer. But, if the peeled areas won't blend and still show after staining then this is the only remaining option.