public static void recordScore(ScoreModel scoreModel)
        {
            var path = "../../Media/Leaderboard/Leaderboard.xml";
            var saveModel = new LeaderboardModel();


            Stream fs = new FileStream(path, FileMode.Open);
            XmlReader reader = new XmlTextReader(fs);
            XmlSerializer serializer = new XmlSerializer(typeof(LeaderboardModel));
            if (serializer.CanDeserialize(reader))
            {
                LeaderboardModel o = (LeaderboardModel) serializer.Deserialize(reader);
                saveModel.ScoreModels.AddRange(o.ScoreModels);
            }
            fs.Close();

            saveModel.ScoreModels.Add(scoreModel);

            System.Xml.Serialization.XmlSerializer writer =
                new System.Xml.Serialization.XmlSerializer(typeof(LeaderboardModel));

            System.IO.FileStream file = System.IO.File.Open(path, System.IO.FileMode.Append);

            writer.Serialize(file, saveModel);
            file.Close();
        }

<?xml version="1.0"?>
<LeaderboardModel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ScoreModels>
    <ScoreModel>
      <Name>Test</Name>
      <GameSpeed>Normal</GameSpeed>
      <Score>11</Score>
    </ScoreModel>
  </ScoreModels>
</LeaderboardModel><?xml version="1.0"?>
<LeaderboardModel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ScoreModels>
    <ScoreModel>
      <Name>Test</Name>
      <GameSpeed>Normal</GameSpeed>
      <Score>11</Score>
    </ScoreModel>
  </ScoreModels>
</LeaderboardModel><?xml version="1.0"?>
<LeaderboardModel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ScoreModels>
    <ScoreModel>
      <Name>Test</Name>
      <GameSpeed>Normal</GameSpeed>
      <Score>11</Score>
    </ScoreModel>
  </ScoreModels>
</LeaderboardModel><?xml version="1.0"?>

static void Main(string[] args)
    {
        LeaderboardModel lm = new LeaderboardModel();
        List<LeaderboardModelScoreModels> m = new List<LeaderboardModelScoreModels>();
        lm.ScoreModels = m;

        for (int i= 0;i < 2;i++)
        {
            m.Add(new LeaderboardModelScoreModels() { ScoreModel = new LeaderboardModelScoreModelsScoreModel() { GameSpeed = "hi", Name = "hi", Score = 1 } });
        }
        System.Xml.Serialization.XmlSerializer writer =
            new System.Xml.Serialization.XmlSerializer(typeof(LeaderboardModel));

        System.IO.FileStream file = System.IO.File.Open("D:\a.xml", System.IO.FileMode.Append);

        writer.Serialize(file, lm);
        file.Close();

    }

using System.Collections.Generic;
  [System.SerializableAttribute()]
 [System.ComponentModel.DesignerCategoryAttribute("code")]
 [System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true)]
 [System.Xml.Serialization.XmlRootAttribute(Namespace = "", IsNullable =
 false)]
 public partial class LeaderboardModel
 {

private List<LeaderboardModelScoreModels> scoreModelsField;

/// <remarks/>
[System.Xml.Serialization.XmlElementAttribute("ScoreModels")]
public List<LeaderboardModelScoreModels> ScoreModels
{
    get
    {
        return this.scoreModelsField;
    }
    set
    {
        this.scoreModelsField = value;
    }
}
}


[System.SerializableAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true)]
 public partial class LeaderboardModelScoreModels
{

private LeaderboardModelScoreModelsScoreModel scoreModelField;

/// <remarks/>
public LeaderboardModelScoreModelsScoreModel ScoreModel
{
    get
    {
        return this.scoreModelField;
    }
    set
    {
        this.scoreModelField = value;
    }
}
}


 [System.SerializableAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true)]
public partial class LeaderboardModelScoreModelsScoreModel
{

private string nameField;

private string gameSpeedField;

private byte scoreField;

/// <remarks/>
public string Name
{
    get
    {
        return this.nameField;
    }
    set
    {
        this.nameField = value;
    }
}

/// <remarks/>
public string GameSpeed
{
    get
    {
        return this.gameSpeedField;
    }
    set
    {
        this.gameSpeedField = value;
    }
}

/// <remarks/>
public byte Score
{
    get
    {
        return this.scoreField;
    }
    set
    {
        this.scoreField = value;
    }
  }
}