#include <File.au3>
Dim $aFileArray, $aLength, $i

_FileReadToArray('source.txt', $aFileArray, 0)
_ArraySort($aFileArray)

$aLength = UBound($aFileArray)
For $i = 0 To $aLength - 1
  $aFileArray[$i] = StringRegExpReplace($aFileArray[$i],'(?<=txt-no=")[0-9]+(?=")', $aLength - $i)
Next
_FileWriteFromArray('output.txt',$aFileArray)