Replacing a shower faucet becomes a necessary task whenever the old faucet wears out or breaks. The key to doing it correctly is to use the right tools and purchase the correct replacement faucet. It is a mistake to assume all faucets are interchangeable. Faucets come in many sizes, types and styles and the replacement should match the original faucet size exactly.Selecting a New FaucetBefore you replace your faucet, you need to take down measurements. Take an exact measurement of the holes for the faucet and write it down. Remove the old faucet and take it to the store with you, so you can compare sizes with other faucets.Using the right tools helps prevent leaks after you install the faucet. Open ended wrenches and adjustable wrenches are ideal for tightening square or hex shaped nuts and adjusting other interior parts of the faucet and corresponding valves. Pipe wrenches, on the other hand, can destroy chrome finish on nuts and pipes.If you are working in a small shower, a spud wrench can be used on the faucet. It is a thin wrench designed to fit into tight places.Installing Your FaucetOnce you have purchased a new faucet, installation starts with removing the rear access panel at the back of the bathtub. Most tubs have a two faucet valve assembly. Pop off the valve cap, loosen the handle screw and remove the handle. Then loosen the set screw and remove the lever handle. Finally remove the cover plate.Switch off the shutoff valves to prevent cold or hot water from running while installing the replacement. Once you turn off the water, remove, repair, or replace cartridge parts. An adjustable wrench can remove the nut locking the cartridge into the valve body. Then use locking pliers to grip the cartridge and pull it straight out.Clean the cartridge and replace the o-ring. Flush out any excess dirt in the valve body with water. Then reinstall the various existing and replacement components and test out the new faucet. If there are no leaks, your job is done.Label each part you remove and lay them down in order of removal on a shop towel so you can replace them in the same order later.