Im trying to turn off my functions key so when i press F5 it actually refreshes, not makes my screen brighter.?

When i try to turn of the function key, since i have a Dell Inspiron 17R, I have no clue if you can still find that model but still. The function keys area is mainly focused in F1-F12 keys, im trying to make The F1-F12 keys work without me having to press the FN(function key) and instead i have to press the Fn key to change the volume, and or screen brightness. Basically tying to switch what the Fn key uses, instead of me pressing Fn to use F5 and wat not, i want to us the Fn key to adjust screen brightness and volume and such. if i haven't made this to confusing please help. i was able to do it before but now i cant figure it out.