Architecture around mongodb with document and subdocument
{
    ApplicationName: "FooApplication",
    Settings : [
          ConnectionString: "localhost@myserver.com",
          UserName: "Admin"
          Password: "Password"
        ]
    }
}
{
    ApplicationName: "BarApplication",
    Settings : [
          ConnectionString: "localhost@myserver.com",
          UserName: "Admin",
          Password: "Password",
          Special: "aaaaaaaaaaaaaa"
        ]
    }
}

public class ApplicationSettings {
    public ObjectId Id {get; set;}
    public string Name {get; set;}
    public Dictionary<string, string> Settings {get; set;}
}

var query = Query.EQ("_id", someIdHere);
var update = Update.Set("Settings.ConnectionString", "NewLocalhost@myserver.com")
collection.Update(query, update)