How do you adjust screen brightness on Acer laptop?

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http://askubuntu.com – I have an Acer Aspire one 5742 laptop. I installed Linux Ubuntu 12.10 on it. I have one problem with it. no matter how I press the Fn+ up or Fn + down keys I cannot adjust the Screen Brightness. It is at its maximum brightness and this drains the laptop battery. I did some googling and found out that you needed to change some settings in the grub file but I don't know exactly what to do. Can anyone help me? (HowTos)