Hello,
I from Colombia and my english is not so good, please try to understand me, :P
tanks!
I have a question about update XML files with flex.
I read that Flex does not have the capability to write or edit a XML by
itself, using the XML methods I can edit the XML (created into the main
application, i cannot edit a external XML) but the changes do not be saved on
the XML file, so, when I refresh my page the values persist with the old values.
Use server compilers like ASP.NET, PHP, and others are the only way to write
or edit the original XML file?

Yes. ...PHP or some server side script is required to save the changes.
There are many examples, so try google to find some that make sense to you.

And your English is just fine. ;-)