[{"scopes":[{"sections":[{_id:1,title:"A"},{_id:2,title:"B"}]},{"sections":[{_id:1,title:"A"},{_id:2,title:"B"}]}]}]

db.collection.update({"scopes.sections._id":1},{$set:{"scopes.$[].sections.$[s].title":"a updated"}},{arrayFilters:[{"s._id":1}]})