first, download the new texture.  Rename texture texture1 (this is assuming that there is only one texture already with the aircraft.  If there are more textures then name it accordingly) put the texture in the aircraft file you want it in.  Then open up the aircraft config file and you will see something like this:

[fltsim.0]
title=Boeing 737-400
sim=Boeing737-400
model=
panel=
sound=
texture=
kb_checklists=Boeing737-400_check
kb_reference=Boeing737-400_ref
atc_id=N737Z
atc_airline=American Pacific
atc_flight_number=1123
ui_manufacturer=Boeing
ui_type="737-400"
ui_variation="American Pacific Airways"
description="One should hardly be surprised that the world's most prolific manufacturer of commercial aircraft is also the producer of the world's most popular jetliner. The 737 became the best-selling commercial jetliner worldwide when orders for it hit 1,831 in June 1987 (surpassing Boeing's own 727 as the previous champ). However, it wasn't always that way\s in the first few years of production, there were so few orders that Boeing considered canceling the program. They didn't, and the airplane has more than proven itself in over three decades of service."

highlight this and copy it, then go to the end of the last flightsim for your aircraft. hit enter twice then paste what you just copied.  Now you will have to change it from flightsim.0 to whatever like flightsim.1 or 2 or 3 just depends on how many texture already come with you aircraft. Note makesure it is the same number that you renamed the texture you downloaded.  Now change the Texture line:  texture=  to Texture=1

Thats pretty much it.  make sure you go through the fltsim and change airline name atc name ect to match whatever texture you have DL'ed  Then save your aircraft config file and thats it 

Hope I dont confuse you to much.  If I did let me know and I will post the exact fltsim file you need

thanks Hagar.  Now I feel like a ROOKIE.  Just kidding, that is awsome.  That is what I was tring to say, it just came out different