Oral abitfungals. The skin needs tp be cultured to determine if there is mixed infection with bacteria and fungus. This requires local treatment with compresses to remove dead skon as well as oral antifungals and possible oral antibiotics. It amy even require intravenous medications to clear infection amd minimize permanent damage.
Dry them out. The best treatment is to dry the feet out and keep them dry. Trench foot is from prolonged exposure to water on the feet above freezing.