There should be a function key with a padlock icon on it. If you use the Fn key with that, it'll lock the function on or turn it off.
Edit: It's one or the other generally. I think you would need customised software to retain the ability of F5. However, having the lock on simply reverses the status so Fn-F5 will then be the refresh option.
There is a padlock on F2, Fn F2 selects lock and unlock but it doesn't seem to do anything? With either option the F keys still work as F keys not as volume etc
Can you not do it through the Control Centre? thats the only place i think it would be possible if you could.
However, I think it is that you have to press the Fn key and then the F keys for the volume (F8, 9 and 10 I believe for mute, down and up respectively).
Fn F2 I think is either the Windows start key lock to stop it accidentally being pressed in games or its the locking of the touchpad.
Take a picture of the icons and we should be able to help you work it out