[{"_id":ObjectId("5a7e395e20a31e44e0e7e284"),"name":"a","address":[{"street":"123","town":"bar"},{"street":"Lower Street","town":"bar"},{"street":"123","town":"foo"}]}]

db.collection.update({"address.street":"123"},{$set:{"address.$[a].town":"HNO"}},{arrayFilters:[{"a.street":"123"}]})