Replacing a water valve in a bathtub is a relatively simple plumbing repair that can be completed in a few hours. The most common type of water valve in a bathtub is a compression valve, which uses a packing nut to create a seal against the valve seat. Over time, the packing nut can deteriorate and begin to leak. If you notice a leak coming from the base of the valve, it is likely that the packing nut needs to be replaced.
There are a few different types of compression valves, so it is important to identify the type of valve you have before you begin repairs. Once you have identified the type of valve, you can purchase a new packing nut at your local hardware store.