Open both the freezer and refrigerator doors, there are 4 screws at the top of the door frame that extend into the grill covering above the doors where the condenser is housed.  Remove all 4 screws, pull the grill open and then use your home vacuum to clean the dust from the condenser coil (it'll be obvious that it needs it).  Close the grill back, replace the screws and the message should go away.