First you have to save the skin. Open AHII, go into offline or an arena. Go to the hanger and right click on the plane you want to skin. Click Save Skin and exit AHII.
Open Windows explorer and click, C:/Program Files/HTC/Aces High II/skins/yourplanename/whatever.bmp
I usually skin spitfires, so mine would be C:/Program Files/HTC/Aces High II/skins/spit5/spit5sd.bmp/spit5wng.bmp
Then, you should be able to edit the skin.