Write To XML On Server

I am working with an XML doc stored on a server…

The contents of the XML read as follows…

I am looking for a way to update the XML using Applescript.
Eg Update the value of “CurrentUserName” to “Daniel”

I have tried the following but Im getting errors.

tell application "System Events"
	set theXMLFile to ""
	tell XML element "BrightSignUserVariables" of contents of XML file theXMLFile
		set value of XML element "CurrentUserName" to "Test"
	end tell
end tell