how to change xml tag at runtime

Hi
i have flash movie that plays flv file and store it's configuration in xml file. one of those configuration is the path of the flv file
<filepath>myfile.flv</filepath>

i have many flv files uploaded and i want to change the path on the xml file based on somthing that user requests like url parameter or any thing.
so it will be
<filepath>anotherfile.flv</filepath>

how to change the path in xml file ?

thanks in advance.