Put a basin under the trap to catch water. Loosen the nuts at the outlet and inlet of the U-bend by hand or wrench and remove it. The trap will slide off the pop-up body tailpiece when the nuts are loose. Keep track of washers and nuts and their orientation by leaving them on the tubes. Unscrew the cap holding the ball-and-pivot rod in the pop-up body and withdraw the ball. Compress the spring clip on the clevis and withdraw the pivot rod from the clevis. A professional like Clerkenwell Plumbers can provide expert help.Remove the pop-up stopper. Then, from below, remove the lock nut on the stopper body. Thrust the stopper body up through the hole to free the flange from the basin, and then remove the flange and the stopper body. Thread the locknut all the way down the new pop-up body, followed by the flat washer and the rubber gasket (beveled side up). Wrap Teflon tape clockwise onto the top of the threaded body. Seal the flange with plumbers putty and place underneath the drain flange. From below, face the pivot rod opening directly back toward the middle of the sink and pull the body straight down to seat the flange. Thread the locknut/washer assembly up and fully tighten the locknut with a suitable wrench. Do not twist the flange so as not to break the putty seal, a Clerkenwell Plumber has the skills and equipment to complete the job. Drop the pop-up stopper into the drain hole so the hole at the bottom of its post is closest to the back of the sink. Put the beveled nylon washer into the opening in the back of the pop-up body with the bevel facing back. Put the cap behind the ball on the pivot rod as shown. Sandwich a hole in the clevis with the spring clip and thread the long end of the pivot rod through the clip and clevis. Put the ball end of the pivot rod into the pop-up body opening and into the hole in the stopper stem. Screw the cap on to the pop-up body over the ball. Put the cap behind the ball on the pivot rod as shown. Sandwich a hole in the clevis with the spring clip and thread the long end of the pivot rod through the clip and clevis. Put the ball end of the pivot rod into the pop-up body opening and into the hole in the stopper stem. Screw the cap on to the pop-up body over the ball.