[{"client":"stackoverflow","MQTT_FIELD":[{_id:1,name:"abc",index:1}]}]

db.collection.update({"client":"stackoverflow"},[{"$set":{"MQTT_FIELD":{"$cond":{"if":{"$eq":[{"$size":{"$filter":{"input":"$MQTT_FIELD","as":"elem","cond":{"$eq":["$$elem.index",2]}}}},0]},"then":{"$concatArrays":["$MQTT_FIELD",[{name:"S1",index:1}]]},"else":{"$map":{"input":"$MQTT_FIELD","as":"elem","in":{"$setField":{"field":"name","input":"$$elem","value":"S1"}}}}}}}}])