first, you need to remove the hex screws on top of you mag. then, use a hammer(or something hard) to hit the bottom plate out, then you should be able to push(from the top) or pull (from the bottom) the internal mag out, but beware of breaking the fake spring sticker, i have teared mine already.
You can also have a look:
http://www.civiliangunner.com/cgf/viewthread.php?tid=86423
