The overflow pipe, as its name implies, prevents the water level in the bathtub from exceeding a safe depth and spilling over the edge of the tub. This pipe sees little stress and wear and while it should last for many years, it won't last forever. When the need to replace this section of plumbing arises, the amount of work involved will depend largely on where and how the bathtub is installed.
Use a screwdriver to remove the faceplate on the overflow. This usually attaches with two screws that extend into the overflow pipe and hold the pipe opening and gasket against the back of the tub's overflow outlet. Remove these screws and the pipe will pull away from the tub.
Cut the existing drain squarely below the "T" joint that joins the drain and overflow pipes with a hacksaw.
Remove the drain basket and remove the lower drainpipe. A threaded drain basket holds most drains in place. The drain basket is the fitting visible from inside the tub with a mesh or crosspiece that prevents large objects entering the drain. Insert an open pair of needle nose pliers into the drain and unscrew it to loosen the fitting. Alternatively, there may be a large round nut holding the drain in place underneath the tub. Use a pipe wrench to remove this nut and remove the drain.
Use a putty knife to remove the old plumber's putty from the drain opening. Clean any remaining residue from the drain and overflow with a clean rag.
Fit the new drain/overflow assembly to the tub. Hold the overflow faceplate in place, and insert and tighten the two retaining screws.
Attach the new drain spout to the existing drain. Tighten the coupler if using a friction fitting. Use primer and the PVC cement for a slip fitting.
Spread a small bead of plumber's putty around the drain opening. Insert the new drain basket into the drain and tighten in place. Some of the plumber's putty will squeeze out around the flange. Use a clean rag to clean this up before using the tub.
The replacement of an overflow and drain assembly is a simple plumbing job. Actually getting to these fittings may require extensive work. In cases where there is no access to the tub's fittings, it may be necessary to cut a panel through the wall at the front of the tub. If this is not possible, you may need to remove the tub completely.
Test drain fittings after installation. Leaks under the tub can cause considerable damage before being detected.