How To Change a Animation Keyframe With Script?

Okay so I'm trying to make a katana combat system and in order to make it work without looking stupid I have to make the katana swing at the same height that the player's camera is looking. So my question is how would I be able to edit an animation so that the Y vector on the Character's arms is different from how it is in the original animation using a script? I already know to use camera.cframe.lookvector to get the Y value I just don't know how to edit a animation keyframe using a script and the dev forum isn't helping a whole lot.