How To Change The System Rating In Windows 8

1.Open Wordpad or Notepad as Administrator.

2.In Wordpad - File > Open
  Then navigate to - C:\Windows\Performance\WinSAT\DataStore\Formal.Assessment (Initial).WinSAT

3.Open the shown  file. i.e - Formal.Assessment (Initial).WinSAT

4.Now Search "systemscore" without quotation.
   Press Ctrl+F to search "systemscore"
   (You will see your current system rating)

5.Now change the value.
   (Maximun you can change is 9.9)

6.Now Check your System Rating.

Done.