[{"basicDetails":{"id":"1","name":"xyz"},"tasks":[{"id":"10","name":"task10","subtasks":[{"id":"120","name":"subTask120","description":"ABC"}]}]}]

db.collection.update({"basicDetails.id":"1"},{"$set":{"tasks.$[tasks].subtasks.$[subtasks].description":"XYZ"}},{"arrayFilters":[{"tasks.id":"10"},{"subtasks.id":"120"}],multi:true})