How to add attribute to existing xml .help me on same

Try something like this:

string scontent = @"<?xml version='1.0'?>
<props>
<specific_criteria>NONE</specific_criteria>
<specific_condition>NEW</specific_condition>
<specific_disposition>
<disposition>NONE</disposition>
</specific_disposition>
<specific_model>NONE</specific_model>
<specific_velocity>NONE</specific_velocity>
<specific_cons_model>NONE</specific_cons_model>
<specific_program>
<program>NONE</program>
</specific_program>
<specific_operation>NONE</specific_operation>
<specific_oem>NONE</specific_oem>
<specific_esn_status/>
<special_criteria/>
<same_disposition>N</same_disposition>
<same_model>N</same_model>
<same_sku>N</same_sku>
<same_esn_status>N</same_esn_status>
<same_cons_model>N</same_cons_model>
<same_condition>N</same_condition>
<same_velocity>N</same_velocity>
<same_receipt>N</same_receipt>
<same_oem>N</same_oem>
</props>"; 

XDocument xdoc = XDocument.Parse(scontent);

var nodes2change =  xdoc.Root.Descendants().Where(x=>x.Descendants().Count() == 0 && (string)x.Value!=string.Empty);
int i = 0;
foreach (XElement x in nodes2change)
{
	i++;
	x.SetAttributeValue("id", i);
}

//xdoc.Save("newFullFileName.xml");

Result:

<props>
  <specific_criteria id="1">NONE</specific_criteria>
  <specific_condition id="2">NEW</specific_condition>
  <specific_disposition>
    <disposition id="3">NONE</disposition>
  </specific_disposition>
  <specific_model id="4">NONE</specific_model>
  <specific_velocity id="5">NONE</specific_velocity>
  <specific_cons_model id="6">NONE</specific_cons_model>
  <specific_program>
    <program id="7">NONE</program>
  </specific_program>
  <specific_operation id="8">NONE</specific_operation>
  <specific_oem id="9">NONE</specific_oem>
  <specific_esn_status />
  <special_criteria />
  <same_disposition id="10">N</same_disposition>
  <same_model id="11">N</same_model>
  <same_sku id="12">N</same_sku>
  <same_esn_status id="13">N</same_esn_status>
  <same_cons_model id="14">N</same_cons_model>
  <same_condition id="15">N</same_condition>
  <same_velocity id="16">N</same_velocity>
  <same_receipt id="17">N</same_receipt>
  <same_oem id="18">N</same_oem>
</props>

Above code needs to be improved. Feel free to change it!

For further information, please see: XElement.SetAttributeValue Method (XName, Object) (System.Xml.Linq)[^]

Tip: remove && (string)x.Value!=string.Empty from Where statement to get result as you expect ;)