If you actually want to change system icons, you'll need to replace the existing ones in framework-res.apk, resign the apk, then push it back to system (adb or a flashable zip, not just copying it there). If you're talking about an app icon, the it's about the same, but you'd need to copy the apk to your computer, make the change, resign it, then just install it as normal or push it to /system/app. If you just want to pull out the icons, that's pretty easy.