The tub drain and stopper assembly in a bathtub is designed to let water flow out of the tub when the stopper is open, and prevent water from draining when the stopper is closed. Over time, the rubber seals on the stopper, the drain flange or both can harden, preventing a tight seal. The stopper and drain can be replaced separately, but it's usually a good idea to replace them both at the same time.
Remove the tub stopper, adapting your strategy to its design. Some stoppers just unscrew from the drain. Others have a knob that must first be unscrewed to access the stopper body, which can then be unscrewed with a screwdriver.
Insert the tub drain wrench into drain body so the prongs of the tool engage the crosspiece at the bottom of the tub drain. Turn the tool counterclockwise to unscrew the drain and remove it from the tub.
Scrape old putty, dirt or calcification away from the drain opening in the tub with a flexible putty knife. Thoroughly clean the opening before installing the new drain assembly.
Knead a long bead of plumber's putty in your hand and shape it into a rope. Wrap the putty around the backside of the new drain body's flange. Press it firmly into position so it won't move during installation.
Insert the drain body into the opening in the tub and turn it clockwise to engage the threads of the drainpipe below the tub. Tighten the drain body with the tub drain wrench. Remove excess putty that squeezes out from under the flange with the putty knife. Be careful to not scratch the tub.
Install the new stopper into the drain body according to the manufacturer's instructions.