updating xml data

Sir/Ma'am,
I'm trying to modify existing XML data.  For example, I have an xml file in the following format:



 
    1255588
    red
    I4
    sedan
    11000
 
 
    388854
    blue
    V6
    coupe
    09999
 
 
    214569
    green
    V8
    truck, full
    05000
 

I also have a form with 1 combobox (e.g vin.text) and 4 text boxes (e.g. color.text, engine.text, type.text, and price.text), which corresponds to the xml nodes. I'm attempting to write a method that will update the xml data, if it corresponds to the vin number. So, if I want to change the price on vin 1255588 from 11000 to 09999, how could I accomplish that? 
heres my code:

        private void button1_Click(object sender, EventArgs e)
        {
   try
   {
    XmlTextReader reader = new XmlTextReader(@"C:\test.xml");
    XmlDocument doc = new XmlDocument();
    doc.Load(reader);
    reader.Close();
    
    //Select the vin node
    XmlNode oldvin;
    XmlElement root = doc.DocumentElement;
                oldvin = root.SelectSingleNode("/cars/car/[vin='" + vin.Text + "']");
    XmlElement newCd = doc.CreateElement("car");
    newCd.SetAttribute("vin", vin.Text);
    
    newCd.InnerXml = "" + vin.Text + "" +
     "" + color.Text + "" +
     "" + engine.Text + "" +
                 "" + type.Text + "" +
     "" + price.Text + "";
     
    root.ReplaceChild(newCd, oldvin);
    //save the output back to the xml file
    doc.Save(@"C:\test.xml");
    this.DialogResult = DialogResult.OK;
   }
   catch (Exception ex)
   {
    Console.WriteLine ("Exception: {0}", ex.ToString());
    this.DialogResult = DialogResult.Cancel;
   }
  }
        }
    }