how repeat specific tag with child after last tag XML

my xml samle:

<Y>
<x>
<PH>
<SQ>3SQ>
<ND>4524115ND>
<SN>545SN>
<PC>545PC>
<PG>54PG>
<PP>5400PP>
<PS>44500PS>
<IS>0IS>
PH>
<BY>
<MH>
<MG>4545MG>
<MP>25000MP>
<MI>15000MI>
<MS>0MS>
MH>
BY>
x>
Y>

i have create text box for each tag and i have one button (save) to create X tag and chiled.

i want add new button "add MH" to add MH tag and child ( MG,MP,MI,MS) after last MH tag

i wang after click button update xml sample this:

<Y>
<x>
<PH>
<SQ>3SQ>
<ND>4524115ND>
<SN>545SN>
<PC>545PC>
<PG>54PG>
<PP>5400PP>
<PS>44500PS>
<IS>0IS>
PH>
<BY>
<MH>
<MG>4545MG>
<MP>25000MP>
<MI>15000MI>
<MS>0MS>
MH>
<MH>
<MG>123MG> i want add this MH tag
<MP>5000MP>
<MI>5000MI>
<MS>0MS>
MH>
BY>
x>
Y>

my code for Button Save is:

private void button1_Click(object sender, EventArgs e){
XmlNode customernode = xmldoc.CreateNode(XmlNodeType.Element, "x", null);
XmlNode tartibnode = xmldoc.CreateNode(XmlNodeType.Element, "SQ", null);
XmlNode datevisitnode = xmldoc.CreateNode(XmlNodeType.Element, "ND", null);
XmlNode bimenode = xmldoc.CreateNode(XmlNodeType.Element, "SN", null);
XmlNode nezamnode = xmldoc.CreateNode(XmlNodeType.Element, "PC", null);
XmlNode pezeshknode = xmldoc.CreateNode(XmlNodeType.Element, "PG", null);
XmlNode mablaghkolnode = xmldoc.CreateNode(XmlNodeType.Element, "PP", null);
XmlNode mablaghbimarnode = xmldoc.CreateNode(XmlNodeType.Element, "PS", null);
XmlNode mablaghbimenode = xmldoc.CreateNode(XmlNodeType.Element, "IS", null);
XmlNode khedmatnode = xmldoc.CreateNode(XmlNodeType.Element, "MG", null);
XmlNode sahmekolnode = xmldoc.CreateNode(XmlNodeType.Element, "MP", null);
XmlNode sahmebimenode = xmldoc.CreateNode(XmlNodeType.Element, "MI", null);
XmlNode sahmebimarnode = xmldoc.CreateNode(XmlNodeType.Element, "MS", null);
XmlNode tagkolnode = xmldoc.CreateNode(XmlNodeType.Element, "PH", null);
XmlNode tagkhedmatnode = xmldoc.CreateNode(XmlNodeType.Element, "MH", null);
XmlNode tagbynode = xmldoc.CreateNode(XmlNodeType.Element, "BY", null);
tartibnode.InnerText = txtsq.Text;
datevisitnode.InnerText = txtnd.Text;
bimenode.InnerText = txtsn.Text;
nezamnode.InnerText = txtpc.Text;
pezeshknode.InnerText = txtpg.Text;
mablaghkolnode.InnerText = txtpp.Text;
mablaghbimarnode.InnerText = txtps.Text;
mablaghbimenode.InnerText = txtis.Text;
khedmatnode.InnerText = txtmg.Text;
sahmekolnode.InnerText = txtmp.Text;
sahmebimenode.InnerText = txtmi.Text;
sahmebimarnode.InnerText = txtms.Text;
tagkolnode.AppendChild(tartibnode);
tagkolnode.AppendChild(datevisitnode);
tagkolnode.AppendChild(bimenode);
tagkolnode.AppendChild(nezamnode);
tagkolnode.AppendChild(pezeshknode);
tagkolnode.AppendChild(mablaghkolnode);
tagkolnode.AppendChild(mablaghbimarnode);
tagkolnode.AppendChild(mablaghbimenode);
tagkhedmatnode.AppendChild(khedmatnode);
tagkhedmatnode.AppendChild(sahmekolnode);
tagkhedmatnode.AppendChild(sahmebimenode);
tagkhedmatnode.AppendChild(sahmebimarnode);
tagbynode.AppendChild(tagkhedmatnode);
customernode.AppendChild(tagkolnode);
customernode.AppendChild(tagbynode);
rootnote.AppendChild(customernode);
xmldoc.Save(xmlpath); }

for Button update this code but not work:

private void button2_Click(object sender, EventArgs e)
{
xmldoc.Load(xmlpath);
rootnote = xmldoc.DocumentElement;
XmlNode khedmatnode = xmldoc.CreateNode(XmlNodeType.Element, "MG", null);
XmlNode sahmekolnode = xmldoc.CreateNode(XmlNodeType.Element, "MP", null);
XmlNode sahmebimenode = xmldoc.CreateNode(XmlNodeType.Element, "MI", null);
XmlNode sahmebimarnode = xmldoc.CreateNode(XmlNodeType.Element, "MS", null);
XmlNode tagkhedmatnode = xmldoc.CreateNode(XmlNodeType.Element, "MH", null);
XmlNode tagbynode = xmldoc.CreateNode(XmlNodeType.Element, "BY", null);
khedmatnode.InnerText = txtmg.Text;
sahmekolnode.InnerText = txtmp.Text;
sahmebimenode.InnerText = txtmi.Text;
sahmebimarnode.InnerText = txtms.Text;
tagkhedmatnode.AppendChild(khedmatnode);
tagkhedmatnode.AppendChild(sahmekolnode);
tagkhedmatnode.AppendChild(sahmebimenode);
tagkhedmatnode.AppendChild(sahmebimarnode);
tagbynode.AppendChild(tagkhedmatnode);
tagbynode.InsertAfter(tagkhedmatnode , tagkhedmatnode);
xmldoc.Save(xmlpath);
}

tank you