如何在C＃上设置XML元素替换？

Question

所以我正在做一个项目，在从文本中获取一些元素后，用XML替换某些元素时遇到了问题

我尝试了很多选项，先删除再添加 XML是这样的：

<?xml version="1.0" encoding="utf-8"?>
<UserAccountData>
  <user>
    <username>Admin</username>
    <password>Partyguest12</password>
    <ID1>1<stats level="50" HP="1000" exp="250000" /></ID1>
  </user>
  <user>
    <username>2</username>
    <password>2</password>
    <ID2>2<stats Level="1" HP="20" exp="0" /></ID2>
  </user>
  <user>
    <username>3</username>
    <password>3</password>
    <ID3>3<stats Level="1" HP="20" exp="0" /></ID3>
  </user>
  <user>
    <username>4</username>
    <password>4</password>
    <ID4>4<stats Level="1" HP="20" exp="0" /></ID4>
  </user>
  <user>
    <username>5</username>
    <password>5</password>
    <ID5>5<stats Level="1" HP="20" exp="0" /></ID5>
  </user>
</UserAccountData>    

public partial class Game : MetroFramework.Forms.MetroForm
{
    private void Save_data_Progress_Click(object sender, EventArgs e)
    {
        int IDcheck = 0;

        XmlDocument User_Data_Exp_Use = new XmlDocument();
        User_Data_Exp_Use.Load("UserData.xml");

        foreach (XmlNode ID_Finder in User_Data_Exp_Use.SelectNodes("UserAccountData/user"))
        {
            IDcheck++;
            if ((Account.Text == ID_Finder.SelectSingleNode("username").InnerText) && (Log_In_Data.PassKey == ID_Finder.SelectSingleNode("password").InnerText))
            {
                break;
            }
        }
        XmlNode User_Path = User_Data_Exp_Use.SelectSingleNode("UserAccountData/user/ID" + IDcheck);
        XmlElement Stats_Set = User_Data_Exp_Use.CreateElement("stats");
        Stats_Set.SetAttribute("Level", Lv.Text);
        Stats_Set.SetAttribute("HP", Hit_Points.Text);
        Stats_Set.SetAttribute("exp",XP.Text);
        User_Path.AppendChild(Stats_Set);

        User_Data_Exp_Use.Save("UserData.xml");
    }
}

运行后，我什么也没得到，也没有得到任何XML错误，说它为NULL