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You should be able to pop up the cover for the sub and access the screws holding it down. Be careful not to break the top black ring since this is what the cover clips into and you will want to replace it when you pull out the sub.
Oh, you can probably sell the sub on ebay for up to 70 bucks or so if its working.
I replaced mine almost immediately after buying my car, and I used a Kicker free-air 10". Unfortunately the guy at the stereo place had to customize the holder, so I don't know what the people there will say about it.
Also, while you're at it a sub amp would be a good idea.