Replace the Stopper and Tailpiece On Your Bathroom Sink 

The stopper and downspout can get rusty or damaged. To replace the entire tailpiece, you'll need a channel-lock plier and some plumber's putty. Put a bucket or towels underneath and loosen the P-trap or extension nut that attaches the tailpiece to the drain. Next, loosen the nut at the bottom of the sink that attaches the tailpiece to the sink. Remove the nut at the back of the tailpiece that holds the stopper actuator to the tailpiece and remove the actuator. Also remove the actuator from the vertical rod that you pull up to close the stopper. Remove the stopper and use your pliers to hold the top removable part of the tailpiece while turning the tailpiece below the sink to unscrew it. Once the top comes off, the bottom should be loose. Remove it from the trap or extension and then down out of the sink. Clean out the sink hole. Transfer the nut and seal from the old tailpiece to the new tailpiece. Unscrew and remove the top of the tailpiece and apply plumber's putty around the top piece. slip the tailpiece up through the sink hole and then down into the trap or extension. Then screw the top onto the tailpiece and start tightening the nut up to the sink. While things are still loose, insert the stopper with the hole at the bottom forward toward the front of the sink. Slip the small nut onto the horizontal stopper actuator rod and insert the actuator into the back of the tailpiece and through the stopper hole. Tighten the nut by hand until the actuator has some resistance to moving the stopper up and down. Also attach the vertical rod to the horizontal rod as instructed by the tailpiece manufacturer. Now you can tighten the large nut under the sink to pull the upper tailpiece down into the sink. Some putty will usually squeeze out the top, and some may also come out the bottom by the nut. The top of the tailpiece should come in contact with the sink. Clean off the putty the squeezed out. Finally, tighten the nuts on the P-trap and any other loose nuts on the drain. Check to make sure the stopper is actuating properly. If it doesn't close or open fully, adjust according to the manufacturer's instructions. Finally, run water through the sink and check for leaks. Tighten the nuts until any leaks seal up. This should not require a tool. If there is a leak after hand-tightening, disassemble and make sure the seal is in place and oriented with the flat side toward the nut.