Currently there's no way to do it in the game. I think a lot of us are hoping they'll add the ability to create and choose a design to use in the future.

You might be able to edit the XML to do it though according to this: https://forums.galciv3.com/465767/page/1/#3551839.

If what you want to do is change the appearance of the fighters launched by your carriers rather than change what the fighters carry, then you're going to want to play with the fighter's entry in ShipClassDefs.XML or the equivalent file for your mod. This is the ShipClassDefs.XML entry for the Terran Alliance Assault Fighter:

  <ShipClass>
    <InternalName>TerranAssaultFighter</InternalName>
    <DisplayName>TerranAssaultFighter_Class_Name</DisplayName>
    <Description>TerranAssaultFighter_Dec</Description>
    <ThumbnailOverride>Temp_Terran_Knight_Alpha_01.png</ThumbnailOverride>
    <ShipHullType>Tiny</ShipHullType>
    <ShipRule>Balanced</ShipRule>
    <ShipDesign>Terran_Knight_01T</ShipDesign>
    <AIShipClass>Interceptor</AIShipClass>
    <StrategicIcon>Knight</StrategicIcon>
    <BlueprintDef>AssaultFighterBlueprint</BlueprintDef>
  </ShipClass>

The line <ShipDesign>Terran_Knight_01T</ShipDesign> is the line that chooses the hull model used for the fighter, if I'm not mistaken. If I were to change that line to <ShipDesign>Terran_Ranger_02T</ShipDesign>, which is the line used in the Terran Ranger class definition, then the Assault Fighter would look like the Terran Ranger design does.

If you want to change what the fighter carries, then you'll have to mess around with the blueprint file, but that won't significantly impact the appearance of the fighters launched by the carriers.