Catalyzes the conversion of UDP-N-acetylglucosamine (UDP-GlcNAc) to UDP and N-acetyl-D-mannosamine (ManNAc). H2O + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N-acetyl-D-mannosamine + UDP kcat is 4.7 sec(-1). Homodimer. Belongs to the UDP-N-acetylglucosamine 2-epimerase family.