A vet is best.
They need to be able to rinse their nares frequently to keep gunk out of them. A head washer is a good idea and you can find reference to that in the Raising Ducklings sticky (link at the top of the duck forum index).
If it is a full blown infection, they will likely need antibiotics, as I wrote above, a vet is best. But if that is not an option, you can get antibiotics at feed stores and I would guess duramycin10 may work.
So I would make sure they can rinse their heads, maybe put a teaspoon of apple cider vinegar in a quart of water for the head washing - it is a natural antimicrobial. If that doesn't clear things up, I would look up the dosage and get antibiotics going.
