Your applet would load the file from the server
http://helpdesk.objects.com.au/java/how-to-load-xml-document-from-url(or you could just load the text if thats all you needed)
Then to save it you would POST it back to the server (eg. to a servlet)
http://helpdesk.objects.com.au/java/how-do-i-send-a-post-request-using-java