First, you'd need to get access to the edges - the drywall or plaster is covering that up. Then, I'd just take a sawsall and chop it up around the drain. Once you get it released from the drain, you can cart the old tub away. Setting a new tub when you don't have access from below takes some skill...once you get things cleaned out, if you can poke an access hole in the wall at the drain end, you might be able to do it yourself. If not, you probably want to pay a pro to set the tub for you.