How to swap CTRL and Fn Keys?

Since it's likely that you can't, you can consider using a remapper to get something you might prefer over your current layout. I personally really like keyd .

If you want to use Fn as Ctrl because you like the position of Fn, then you're out of luck. That key likely cannot be rebound.

If you want Ctrl to act as Fn, then likely you can: Fn+key sends an output that you can monitor with keyd -m. You can then rebind Ctrl+key to return that output.

On top, you can consider using CapsLock as Ctrl. It's a very, very nice position for Ctrl, I'd say.

I think your best bet is looking for a bios option. Thinkpads have that, I'm not sure about your Dell laptop though

The Fn key is not a key exposed to software, so anything that has to do with it has to be done at the firmware level. ThinkPads have a setting in the bios, maybe your Dell has one too.

Not a perfect solution, but if you're just looking to rebind certain system level commands, you can do that in Settings > Keyboards and at the bottom there's a heading "Keyboard Shortcuts" with a button "View and Customize Shortcuts" where you can change the keybindings of system commands to use CTRL instead of FN.

I guess that's because Fn isn't actually a key but a hardware modifier that allows the keyboard to send a different set of commands e.g. mediakeys.

Unless your system can actually see you hitting Fn alone there would be nothing you could do.