Re: How to downgrade Lumia 520 from amber to stock
don't.
1. Amber, Black, Cyan, whatever is stock. Those are all stock. There is no such thing as custom for WP8 Lumias which implies custom ROMs a la CyanogenMod on Android.
2. I might be mistaken but don't downgrade for a wallpaper.
3. I don't think you can anyway. If anything, you need to upgrade the software.