You'll have to remove some of the drywall around it to reveal the edges. Unscrew it or pull out the nails. Disconnect the fixture and drain, then you should be able to drag it out. BUT, I'd take a sawsall and cut it into more manageable sized pieces. In nearly all cases, the one piece systems are put in place once the house is closed in and before most framing is done - they are just too unwieldy to try to maneuver into place otherwise. Make sure you don't cut any wires or pipes in the demolition process.