Linq to XML - writing XML back to source

I am using Linq to XML. At present i am getting an XML Response from a third party service. The problem i have here is i cant convert some of the data as its been entered in a bad format (See this URL for my current code and a sample of the XML http://forums.asp.net/t/2020186.aspx?Change+date+format+in+Linq).
So now im thinking of saving the XML into a local stringbuilder variable then replacing all the values that are badly formatted and the saving this local variable with the correct info .
Question is how could i write back the values to the relevant node/record using Linq to XML (I already have all the values stored in an array that i want to change but need a way to save it back). My current code to get to the element im interested in is
var ModifiedBeginDate = XDoc.Descendants(ns + "custo ...