If the file follows INI file format rules, then ReadProfileString and WriteProfileString can do the job. Use Read to get the current value and determine if it should be changed, use Write to update it.

If it's a standard text file, then the FileIO UDF might be useful.
$array=FileIO($filename, 'R') reads the file into an array. You can then enumerate the array elements looking for that line, replacing the data when you find it. Then, use FileIO again to write the data back to the file.
 Code:
$Filename='\%userprofile%\AppData\LocalLow\Sun\Java\Deployment\deployment.properties'
$Search='what to look for...'
$aFile = FileIO($Filename, 'R')
For $L = 0 to UBound($aFile)
  If InStr($aFile[$L], $Search)  ; found it - replace the data

  ; choose one of these
    $aFile[$L] = Split($aFile[$L], '=')[0] + '=' + 'new value'
    ; or - to add to the existing data
    $aFile[$L] = $aFile[$L] + 'added value...'

    $L = 99999  ; stop the search
  Next
Next
$ = FileIO($Filename, 'W', $aFile)  ; put the data back in the file
@SERROR ?
This is untested and needs to be tailored to your requirements. If you need to ADD some data, say 5 lines, extend the array and add your data to the new array elements before writing the data back.

Glenn
_________________________
Actually I am a Rocket Scientist! \:D