foreach(string url in records)
{
    // Select a document created today with this url.
    var mongoQuery = Query.And(
                         Query.And(Query.GTE ("created_on", DateTime.Today),
                                   Query.LT  ("created_on", DateTime.Today.AddDays(+1))),
                         Query.EQ ("url", url);

    // now update "isTopApp" field, or create it if doesn't exists already .
    updateStatement = Update.Set ("isTopApp", true);
    bulk.Find (mongoQuery).Upsert().UpdateOne(updateStatement);
}
bulk.Execute();