Open the aircraft.cfg within the aircraft's main folder (using MS Notepad). Look for an entry such as below:
[fltsim.0]
title=B52HUS
sim=B52H
model=
panel=
sound=
texture=SAC
ui_variation=USAF
ui_manufacturer=Boeing
ui_type=B-52H
atc_airline=
description=
atc_id=
OK, so I fictition-alized it a bit to try to get something close to what you may be looking at. The main thing to be concerned with is what follows the "title=" entry. Copy & paste the bolded part EXACTLY as it is into your aircraft.txt folder (which you already know about). Aircraft # ("335") denotes which aircraft follows which flightplan - if you open your flightplans.txt document, look for "AC#335," and you will find (and can edit if desired) the flightplans designated for this particular aircraft. You can of course change the aircraft # but then the corresponding aircraft# in flightplans.txt needs to be changed to match it.