Remove mold from wood - how?

Push a screwdriver into it first. If the wood is soft, that's rot and it needs cutting out, cleaning won't save it. If it's still sound, vacuum the surface (dry brushing just throws spores around), then scrub with a bit of detergent in water and dry it fast. For staining that stays on bare wood, white vinegar sprayed on and left an hour usually lifts it. Mask on while you do it. And find the damp that caused it, or you'll be doing this again next winter.