Remove the two screws that secure the drain assembly's coverplate to the wall of the bathtub with a screwdriver.
Pull the drain assembly out of the wall of the bathtub to expose the stopper at the bottom of the assembly's linkage.
Compress the two ends of the cotter pin that secures the stopper to the bottom of the assembly with pliers, and pull the pin out of the assembly to release the stopper.
Coat the O-rings that wrap around the replacement stopper with heatproof grease.
Position the stopper against the drain assembly's linkage and slide the cotter pin through the stopper and linkage.
Separate the two ends of the cotter pin with pliers to secure the stopper to the linkage.
Lower the stopper into the wall of the bathtub, along with the drain assembly.
Position the drain assembly against the wall of the bathtub and tighten the coverplate's two screws into the assembly with a screwdriver.