Catalyzes the last step of neomycin B biosynthesis, i.e. the irreversible epimerization at C-5''' of neomycin C to give neomycin B. To a lesser extent, is also able to convert neomycin Y2 to neomycin Y1. AH2 + neomycin C + S-adenosyl-L-methionine = 5'-deoxyadenosine + A + H(+) + L-methionine + neomycin B Binds 2 [4Fe-4S] clusters. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. The second cluster is proposed to be involved in the electron-transfer mechanism for the catalytic cycle. Antibiotic biosynthesis; neomycin biosynthesis. Belongs to the radical SAM superfamily.