#include <FileConstants.au3>

$XML = "C:\Dump\SmartConsole.LoginParams.xml"
$txt = FileRead($XML)
$res = StringRegExpReplace($txt, '(<Username>aa</Username>\s+)(\N+)', "$1<Username></Username>")

$file = FileOpen($XML, $FO_OVERWRITE)  ; <<< Write mode (erase previous contents)
FileWrite($file, $res)
FileClose($file)