you can either access ace from within fsx  Add-ons -> CaptainSim -> 757 Ace and change there, followed by reloading the model(make sure you remove read-only option in CS_B757-200 folder's properties to make this work)  

OR

manually change aircraft.cfg file within folder

C:/program files/Microsoft Games/Microsoft Flight Simulator X/ SimObjects/ Airplanes/ CS_B757-200/

for example you want to show winglets in first plane listed in that file change the red entry model=PW to model=PWW

[fltsim.0]
title=Captain Sim 757-200 FSX 0
sim=cs757-200
model=PW
panel=
sound=PW
texture=delta
kb_checklists=
kb_reference=
atc_id=N675DL
atc_airline=Delta
atc_flight_number=757
atc_heavy=0
atc_parking_types=
ui_manufacturer=Boeing
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
ui_type=757-200 PW
ui_variation=Delta
description='757-200 Captain' Pro Pack v.4.1\n(c) 2008 Captain Sim \nwww.captainsim.com

after changing it should look like this :

[fltsim.0]
title=Captain Sim 757-200 FSX 0
sim=cs757-200
model=PWW
panel=
sound=PW
texture=delta
kb_checklists=
kb_reference=
atc_id=N675DL
atc_airline=Delta
atc_flight_number=757
atc_heavy=0
atc_parking_types=
ui_manufacturer=Boeing
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
ui_type=757-200 PW
ui_variation=Delta
description='757-200 Captain' Pro Pack v.4.1\n(c) 2008 Captain Sim \nwww.captainsim.com

incase the model you wanna use has Rolls Royce engines it should similarly be changed from model=RR to model=RRW