By default, the top row of keys on your K400 Plus keyboard are assigned to navigation and media controls. You can set the keyboard to only use standard F1 - F12 functions using Logitech Options software.
 
NOTE: If you don’t have Logitech Options, you can download it from the Wireless Touch Keyboard K400 Plus Downloads page.

To set the function keys to use only standard functions:

1. Start Logitech Options: 
   Start > All Programs > Logitech > Logitech Options
2. Select the Keyboard tab in the upper left corner of the window.
3. In the bottom left hand corner of the window, check Use F1-F12 as standard function keys. Your selection is automatically saved.