I have a Fujitsu C Series 2340 laptop. How do I unlock the function keys?

Define unlock?

"Unlock" means the FN keys are automatically displayed when typign without holding down the FN key + another key.

For example, when I hit the letter "I", the number "3" is displayed without holding down using (FN key + 3)

I don't think there is a way to lock the Fn key like you can the Caps lock key. But if you want to use the keyboard as a numerical keypad, use the Num Lock key. Then the numbers on the letter keys will show.