It is replaced from the top, that is sitting in the tub. Hopefully you have *crosshairs* or a couple of little *nibs* inside the drain flange (the chrome part). That's the part that unscrews. The tool is called a *pickle* - it has a fork at one end and crossed slots at the other. Or a *dumbell* which is tapered and has crossed slots at both ends. Or just use pliers and stick the handle end down into the drain, catch the cross hairs or nibs and unscrew. Clean off the old plumbers putty. Slide a new washer between the underside of the tub and the *shoe* (part with female threads) and put putty around the chrome flange and screw it back in.