Write ti XML with C#



I am trying to collect and write some temporary data within xml file
before I put them into DB.
I am writing into xml file like follows:
private void WriteXML()
{
//Use the xmlTextWriter to open a new xml file
XmlTextWriter writer = new
XmlTextWriter(@"C:\\Data\\tblPomocnaTablica.xml",
System.Text.Encoding.UTF8);

//Write the root element
writer.WriteStartElement("tblTableX");

// Write the elements
writer.WriteElementString("ClientID", "2");
writer.WriteElementString("EmployeeID", "4");
writer.WriteElementString("ShopID", "100");
writer.WriteElementString("ProductID", "100");
writer.WriteElementString("Qty", "1");
// end the root element
writer.WriteEndElement();
// Flush
writer.Flush();
//Write the XML to file and close the writer
writer.Close();
}
It is works and I get something like this:
ClientID | EmployeeID | ProdavonicaID | ProductID | Qty |
2 | 4 | 100 | 100 |1 |
Two Questions
1) How I can write a new rows without rewriting existing rows,
for e.g.?
2 | 4 | 124 | 25 |1 |
etc.

Result should be something like this:
ClientID | EmployeeID | ProdavonicaID | ProductID | Qty |
------------------------------------------------------------------------
2 | 4 | 100 | 100 |1 |
2 | 4 | 124 | 25 |1 |

2) How I can change one value in some row, for e.g.
row 2 ShopID = 250, is that possible?
Thanks in advance
Mike
.