keyboard - Enable F1-F12 keys without Fn on Lenovo Ideapad S400u - Super User

I think the Lenovo IdeaPad S300/S400/S400u/S405 User Guide V2.0 (pdf) has what you want on page 12 in Chapter 2. Learning the basics where it says:

• Setting hotkey mode
  
  By default, the hotkey functions are accessible by pressing the appropriate hotkey. However, you can disable the hotkey mode in the BIOS setup utility.
  To disable the hotkey mode:
  
  
  1. Shut down the computer.
  2. Press the Novo button and then select BIOS Setup.
  3. In the BIOS setup utility, open the Configuration menu, and change the setting of HotKey Mode from Enabled to Disabled.
  4. Open the Exit menu, and select Exit Saving Changes.

Read full article from keyboard - Enable F1-F12 keys without Fn on Lenovo Ideapad S400u - Super User