How to modify inner xml in c#?

public static bool SaveXmlInfo(Sales1 sales1)
{
   int recordsAffected = 0;

    string orderXml = string.Empty;

    StringWriter output = new StringWriter();

    //Seriealize class object into XML type.

    XmlSerializer xs = new XmlSerializer(salesPurchase.GetType());

    xs.Serialize(output, salesPurchase);

    orderXml = Convert.ToString(output);

    //Converting string type XML data into XML format.          

    XElement planPropertyElement = XElement.Parse(orderXml);

//HERE I WANT TO MODIFY INNER XML WHICH IS AVAILABLE IN planPropertyElement VARIABLE ELEMENT //NAME,ITS ATTRIBUTE NAME,DELETE ATTRIBUTE AND SO ON….HOW I MODIFY THIS XML PART……………..

   using (SqlConnection conn = new SqlConnection(ConnectionString1))

    {

        conn.Open();

        SqlParameter[] parms = new SqlParameter[2];

        parms[0] = new SqlParameter("@RCustId", sales1.RCustId);

        parms[1] = new SqlParameter("@SesId", sales1.SesId);

        parms[2] = new SqlParameter

        {
            ParameterName = "@CenXml",

            SqlDbType = SqlDbType.Xml,

            Value = new SqlXml(planPropertyElement.CreateReader())

        };

recordsAffected = SqlHelper.ExecuteNonQuery(conn, CommandType.StoredProcedure, "usp_PurchXml", parms);

        salesPurchase = null;

    }

    return recordsAffected > 0 ? true : false;

}