I'm trying to use a downloaded cabin announcement pack. Every time I edit the IATA code from FSL to SWR and save it, it always reverts back to FSL. Can anyone provide assistance to solve this issue?

Where are you doing this editing?  You do realise if you say “aircraft.cfg” I am going to point you in the direction of a manual since that kind of editing doesn’t work around here  

The instructions says "You livery must be containing the "fltsim" text file in which it should be having the following line
for the sounds to work properly "airline_iata_code=SWR" The same applies for any other livery depeing upon the airlines IATA code". If this is not the aircraft.cfg, then can you point me in the right direction?

The fltsim.cfg is in the texture folder of the livery.