What's the most efficient way to locate and set element values in an XDocument?

<Request module="CRM" call="list_service_features" id="{ID}">
  <block name="auth">
     <a name="username" format="text">{USERNAME}</a>
     <a name="password" format="password">{PASSWORD}</a>
     <a name="client-id" format="counting">{CLIENT-ID}</a>
  </block>
  <a name="service-id" format="counting">{SERVICE-ID}</a>
</Request>

List<XElement> e = doc.Descendants("a").ToList();
e.Where(x => x.Attributes("name") == "username").Single().Value = "abc";
e.Where(x => x.Attributes("name") == "password").Single().Value = "abc";

string xmlInput = ...;
XDocument myDoc = XDocument.Parse(xmlInput);
//
List<XElement> someElements = myDoc.Descendants("a").ToList();
someElements.ForEach(x => x.Value = "Foo");
//
Console.WriteLine(myDoc);

string xmlInput = //...
XDocument myDoc = XDocument.Parse(xmlInput);
//
List<XText> someText =
  myDoc.Descendants()
  .Nodes()
  .OfType<XText>()
  .Where(x => x.Value.StartsWith("{") && x.Value.EndsWith("}"))
  .ToList();
//
List<XAttribute> someAttributes =
  myDoc.Descendants()
  .Attributes()
  .Where(x => x.Value.StartsWith("{") && x.Value.EndsWith("}"))
  .ToList();
//
someText.ForEach(x => x.Value = "Foo");
someAttributes.ForEach(x => x.Value = "Bar");
//
Console.WriteLine(myDoc);

List<XElement> e = myDoc.Descendants("a").ToList();
e.Where(x => x.Attribute("name").Value == "username").Single().Value = "abc";
e.Where(x => x.Attribute("name").Value == "password").Single().Value = "abc";