How can I remove mold from the underside of garage roof sheathing?
Sounds like you’re the “seller” and the “buyer’s” inspector found the problem. 

In real estate sales, I think it is better for the seller to reduce the asking price and let the buyer “fix” the problem. If you try to fix the problem, I can guarantee you that it won’t be good enough and you’ll be re-negotiating the cost and repair later.