How to invert the Fn key

On my Dell XPS 15, it seems to be as simple as me accidentally pressing the Fn Lock button behind the Escape key. If you have this problem, simply hit Fn+Esc and it should revert back to normal, without having to go into the BIOS

That's probably due to an extra option in the BIOS made by the manufacturer. When booting press F2 (usually) to get into the BIOS settings and there you can revert back to function keys instead of multimedia.

Have you tried to press the numbers lock key to change the fn keys functions?

On my Dell laptop I pressed Fn + Esc and the problem is solved for me. If you closely notice, there is a small lock (Fn written on it) sign on the Esc. Apparently, you can change the default behavior of Fn key using Fn + Esc.