Catalyzes the reversible epimerization of cellobiose to 4-O-beta-D-glucopyranosyl-D-mannose (Glc-Man). Can also epimerize cellotriose to Glc-Glc-Man, cellotetraose to Glc-Glc-Glc-Man, lactose to epilactose, and mannobiose to 4-O-beta-D-mannopyranosyl-D-glucopyranose (Man-Glc). May function as a mannobiose 2-epimerase in vivo and be involved in a mannan catabolic pathway which feeds into glycolysis. D-cellobiose = beta-D-glucosyl-(1->4)-D-mannopyranose kcat is 67.6 sec(-1) for cellobiose. kcat is 79.5 sec(-1) for lactose. kcat is 104 sec(-1) for mannobiose. Optimum pH is 7.5. Optimum temperature is 45 degrees Celsius. Belongs to the cellobiose 2-epimerase family.