There are similar posts, but those are for different laptop. I am using Dell Inspiron N5010. I couldn't find how to invert fn keys either from BIOS or from Windows Mobility Center.
How to invert function keys on Dell N5010?
are you sure you did not miss it?
See page 36 of this document
All your Docs for that Model
I did it using BIOS option, enter the BIOS, in it there is one slot for the function key. Reverse it to multimedia key, save and exit.