Catalyzes the conversion of UDP-N-acetylglucosamine into UDP-N-acetylmannosamine, a precursor of the teichoic acid linkage unit. UDP-N-acetyl-alpha-D-glucosamine = UDP-N-acetyl-alpha-D-mannosamine Cell wall biogenesis; poly(glycerol phosphate) teichoic acid biosynthesis. Belongs to the UDP-N-acetylglucosamine 2-epimerase family.