[{"_id":{"$oid":"638ecf5247cf747bdd862bfb"},"title":"This is title one"},{"_id":{"$oid":"638ecf5247cf747bdd862bfc"},"title":null},{"_id":{"$oid":"638ecf5247cf747bdd862bfd"},"title":"This is title three"}]

db.collection.update({"title":null},{$set:{title:"not-found"}})