Catalyzes the reversible epimerization at C-2 of UDP-N-acetylglucosamine (UDP-GlcNAc) and thereby provides bacteria with UDP-N-acetylmannosamine (UDP-ManNAc), the activated donor of ManNAc residues. UDP-N-acetyl-alpha-D-glucosamine = UDP-N-acetyl-alpha-D-mannosamine Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Homodimer. Belongs to the UDP-N-acetylglucosamine 2-epimerase family.