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I do have a wireless Logitech K750 keyboard (silver Mac version) connected to a Lenovo ThinkPad (via the unifying USB receiver, together with a wireless Logitech mouse M705) and currently running Ubuntu 12.04.

By default the F-keys are mapped as multimedia/function keys, e.g. Play /Pause. As I don't use them very often, I want the F-keys to behave like the "real" F-keys (F1-F12). How can I do that?

I know there is an option in the BIOS settings, but this only toggles the built-in keyboard of the notebook, not on my wireless keyboard.

I know there is a way to change this for the Apple wireless keyboard (AppleKeyboard#Change_Function_Key_behavior), but there is no "parameters" directory in /sys/module/hid_logitech_dj/.

Any ideas? Or is there maybe a way to do this in a later Ubuntu version (13.04)?

UPDATE:

lsusb -d 046d: shows
Bus 002 Device 005: ID 046d:c52b Logitech, Inc. Unifying Receiver

UPDATE 2:
Thanks a lot for your help Lekensteyn. Finally Solaar works just fine. In the "Known Issues" of Solaar it says that it does not work with the Nano Receiver. But I have one and it works just fine. Just don't forget to read the installation instructions and run the rules.d/install.sh script. It also recognized my M705 Mouse.

Solaar is a graphical utility that has the ability to configure the Fn key functionality on newer devices:

Since Ubuntu 15.04 (Vivid), Solaar can be installed from the repositories (sudo apt-get install solaar). In previous versions, it is available though the ppa:daniel.pavel/solaar PPA (if you do not know how to install such a "PPA", see What are PPAs and how do I use them?). After installation, you can search for Solaar in the applications menu and start it from there.

If you prefer a command line tool, that is also available! Example usage:

$ /opt/extras.ubuntu.com/solaar/bin/solaar-cli config fn-swap

fn-swap = False

$ /opt/extras.ubuntu.com/solaar/bin/solaar-cli config fn-swap on

fn-swap = True

Note that these settings are not persistent, you will loose it after reboot. This may change in future versions of Solaar.

I have previously created the ltunify tool which is a Unifying tool that is able to communicate in the older HID++ 1.0 protocol. Until the next version of Solaar is released (current version is 0.8.7), you can use ltunify to toggle the Fn key on an older device such as the K800 keyboard.

Installation instructions (taken from https://askubuntu.com/a/114089/6969):

sudo apt-get install git gcc

git clone https://git.lekensteyn.nl/ltunify.git -b fkeyswap

cd ltunify

make install-home

The command ltunify fkeyswap shows the current status:

$ sudo ltunify fkeyswap 

F key functions are not swapped

To swap the functionality, run ltunify fkeyswap on:

$ sudo ltunify fkeyswap on

F key functions are now swapped

There is also an off action:

$ sudo ltunify fkeyswap off

F key functions are now normal

Please leave a comment if it does not work so it can be fixed. The above functionality uses undocumented hardware registers.

You no longer need a ppa to install this in Ubuntu 15.08 wily. Just type

sudo apt-get install solaar

at the command line.

Additionally, if you are using Unity or Gnome Shell, you can use the following command instead:

sudo apt-get install solaar-gnome

For wired keyboard like k290 use https://github.com/milgner/k290-fnkeyctl
and execute command

/usr/local/sbin/k290_fnkeyctl

Solaar can connect only to Unifying Receiver, but not through Bluetooth.

I have K780 keyboard. I observed that if I pair to different Windows machines, using the Logitech Options I have to set FN key swap individually.
Therefore I paired to [2] a windows machine just to be able to use Logitech Options to swap FN keys, then I paired again [2] to my Ubuntu Laptop. The FN swap made earlier is not persisted in [2].

Therefore currently I have no way to swap FN keys from Ubuntu connected through Bluetooth.
I was looking into the github repo for Solaar, there I found the documentation about the register regarding FN swap, but I couldn't find yet a solution to set that register through Bluetooth from linux.