I don't think your refrigerator has a defroster or defrost mode. It would require a heating element in the freezer, which the manual shows no evidence. There is a divider heater to reduce condensation to the gaskets between the two boxes. Also a heater for the ice maker water line.
If your freezer didn't ice up in the past but does now, most likely the gasket has failed allowing air into the freezer compartment. Check that the door closes properly and adjust if needed, also you may look to buy a new gasket, although some refrigerator models require a new door, the gasket isn't sold separately. (I don't know if yours falls in that category)