How can I update these XML values if they have been updated, when parsing XML in C#?

I am parsing XML to update if changes are made. A check is done to see if the order already exists, and if so, doesn't add the invoice rows. However, how can I go about modifying the values if they are updated? Since these are nodes within nodes and whatnot, I have also added the XML example here, with the included invoiceRows.

XML

<?xml version="1.0" encoding="UTF-8"?>

-<InvoiceOrder xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<OrderId xmlns="http://24sevenOffice.com/webservices">88844165</OrderId>

<CustomerId xmlns="http://24sevenOffice.com/webservices">7</CustomerId>

<CustomerName xmlns="http://24sevenOffice.com/webservices">Test Company</CustomerName>


-<Addresses xmlns="http://24sevenOffice.com/webservices">


-<Invoice>

<Street>Chestnut Avenue</Street>

<State>Hampshire</State>

<PostalCode>SO53 ABC</PostalCode>

<Name>Test House</Name>

<City>Eastleigh</City>

<Country>UK</Country>

</Invoice>


-<Delivery>

<Street> 56 Test Street</Street>

<PostalCode>BL4 8BE</PostalCode>

<Name>Mrs Hughes</Name>

<City>bolton</City>

<Country>UK</Country>

</Delivery>

</Addresses>

<OrderStatus xmlns="http://24sevenOffice.com/webservices">Invoiced</OrderStatus>

<DateOrdered xmlns="http://24sevenOffice.com/webservices">2015-11-02</DateOrdered>

<PaymentTime>30</PaymentTime>

<IncludeVAT xmlns="http://24sevenOffice.com/webservices">True</IncludeVAT>


-<Currency xmlns="http://24sevenOffice.com/webservices">

<Symbol>LOCAL</Symbol>

</Currency>

<TypeOfSaleId xmlns="http://24sevenOffice.com/webservices">-100</TypeOfSaleId>


-<InvoiceRows xmlns="http://24sevenOffice.com/webservices">


-<InvoiceRow>

<ProductId>H1023</ProductId>

<RowId>1</RowId>

<Price>708.93</Price>

<Name>CADENCE RAD</Name>

<DiscountRate>0.0000</DiscountRate>

<Quantity>1</Quantity>

<Cost>0.0000</Cost>

<InPrice>0.0000</InPrice>

</InvoiceRow>


-<InvoiceRow>

<ProductId>H1206</ProductId>

<RowId>2</RowId>

<Price>188.81</Price>

<Name>DAISY 6 TOWEL WARMER</Name>

<DiscountRate>0.0000</DiscountRate>

<Quantity>1</Quantity>

<Cost>0.0000</Cost>

<InPrice>0.0000</InPrice>

</InvoiceRow>

</InvoiceRows>

</InvoiceOrder>

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C# Code

bool orderidfound = false; //checks whether order already exists

                                                    for (int k = 0; k < po.Length; k++)
                                                    {
                                                        if (po[k].OrderId == Convert.ToInt32(fileref.importList["OrderId"].InnerText)) //if order number is equal
                                                        {
                                                            orderidfound = true;
                                                        }
                                               
                                                    }

                                                    if (orderidfound == false) //ensures duplicate invoice rows are not added
                                                    {

                                                        newInvoice.InvoiceRows = order.Elements(ns + "InvoiceRows")
                                                        .Elements(ns + "InvoiceRow")
                                                        .Select(e => new Invoices.InvoiceRow
                                                        {

                                                            ProductId = e.Element(ns + "ProductId") == null ? 0 : (int)e.Element(ns + "ProductId"),
                                                            RowId = e.Element(ns + "RowId") == null ? 0 : (int)e.Element(ns + "RowId"),
                                                            Price = e.Element(ns + "Price") == null ? decimal.Zero : (decimal)e.Element(ns + "Price"),
                                                            Name = e.Element(ns + "Name") == null ? string.Empty : (string)e.Element(ns + "Name"),
                                                            DiscountRate = e.Element(ns + "DiscountRate") == null ? decimal.Zero : (decimal)e.Element(ns + "DiscountRate"),
                                                            Quantity = e.Element(ns + "Quantity") == null ? decimal.Zero : (decimal)e.Element(ns + "Quantity"),
                                                            Cost = e.Element(ns + "Cost") == null ? decimal.Zero : (decimal)e.Element(ns + "Cost"),
                                                            InPrice = e.Element(ns + "InPrice") == null ? decimal.Zero : (decimal)e.Element(ns + "InPrice"),
                                                            ChangeState = invState,
                                                        })
                                                        .ToArray();
                                                    }

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