On resident evil 4 for the wii how do you change Ashley's outfit?

Answer 1

After completing the game once, you will unlock an option that allows you to change what Leon and Ashley wear (Police uniform and gangster outfit for Leon and a knights armour or casual clothes for Ashley).

Answer 2

You do play as her in chapter 3-4. But that's the only time you can play as her throughout the game.

The only other way you can play as her is using GameShark or Action Replay.