using System;

using System.Collections.Generic;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

public class Program

{

    public class FormElement

    {

        public Int32 FormElementId { get; set; }

        public String FormElementName { get; set; }

    }

​

    public class FormPageElement

    {

        public Int32 FormPageElementId { get; set; }

        public Int32 FormElementId { get; set; }

    }

    public static void Main()

    {

        String xmlString = "<FormData>" +

            "  <Info UserCreated=\"GUID\" />" +

            "  <FormElements>" +

            "   <FormElement formElementId=\"1\" fieldTypeId=\"1\" text=\"aaa\" />" +

            "   <FormElement formElementId=\"2\" fieldTypeId=\"2\" text=\"bbb\" />" +

            "   <FormElement formElementId=\"3\" fieldTypeId=\"3\" selectedIndex=\"\" selectedText=\"\" />" +

            "   <FormElement formElementId=\"4\" fieldTypeId=\"4\" time=\"\" />" +

            "   <FormElement formElementId=\"5\" fieldTypeId=\"5\" date=\"\" />" +

            "   <FormElement formElementId=\"6\" fieldTypeId=\"6\" signatureImageData=\"\" signaturePoints=\"\" />" +

            "   <FormElement formElementId=\"7\" fieldTypeId=\"7\" latitude=\"\" longitude=\"\" />" +

            "   <FormElement formElementId=\"8\" fieldTypeId=\"8\" imageData=\"\" imageFilepath=\"\" annotationImageData=\"\" annotationPoints=\"\" />" +

            "   <FormElement formElementId=\"9\" fieldTypeId=\"1\" text=\"ccc\" />" +

            "  </FormElements>" +

            "</FormData>";

​

        XDocument xDoc = XDocument.Parse(xmlString);

​

        Int32 formElementId = 2;    

        String text = "foo";

        XElement element = xDoc.Descendants().Elements("FormElement").Single(x => x.Attribute("formElementId").Value == formElementId.ToString());

        element.SetAttributeValue("text", text);

        Console.WriteLine(xDoc.ToString());

    }

}