<BankName BankName="jinesh" TemplateModel="sam">
<ChqBasics>
  <ChqL>30</ChqL>
  <ChqW>50</ChqW>
  <ChqImgPath>hai</ChqImgPath>
</ChqBasics>
<XandYPosition>
  <column Name="ChqDate" X="10" Y="10" />
  <column Name="CPayAgainst" X="10" Y="10" />
  <column Name="ChqAmtDgt" X="10" Y="10" />
  <column Name="ChqAmtWrds" X="10" Y="10" />
</XandYPosition>

public static void updatenode(string bankname, string templatemodel,string field,string x,string y)
    {
        XDocument doc = XDocument.Load("newtest.xml");
       var  updatenode = doc
                    .Descendants("BankName")
                    .Where(item => item.Attribute("BankName").Value == bankname && item.Attribute("TemplateModel").Value == templatemodel)
                    .Select(XandYPosition => XandYPosition.Descendants("XandYPosition").Descendants());
    }

BasicClass.updatenode(comboBox1.Text, comboBox2.Text, "ChqDate", "1000", "2000");