The caulking in your shower keeps water away from the wall and prevents mildew build-up. When the caulking develops cracks or becomes covered in mildew, you'll probably want to replace it. Caulking won't stick to old caulking, so you'll need to remove all traces of the old caulking first and clean the area thoroughly.
Things You'll Need
Bathroom or tub and tile cleaner
Heat gun or hair dryer
Bleach or other mildew-cleaning product
Clean the area around the caulking thoroughly. You need a clean surface when you recaulk, and cleaning after you remove the old caulking will introduce water into the exposed seam. Make sure to dry the area thoroughly before you move on to the next step.
Determine what type of caulking you're working with. Cut the caulking with a sharp utility knife. If it feels soft and rubbery, it's likely silicone caulk. If it's hard, then it's likely a PVC- or water-based caulk.
Use the low setting of a heat gun or a hair dryer to soften PVC- or water-based caulk. Be careful not to overheat any areas--you want to soften the caulk, not burn it. There are also some chemical caulk-softening products on the market that you can use instead. It may take up to several hours for the caulk to soften after applying the softening agent to the seam.
You won't need heat to remove silicone-based caulk.
Use a utility knife to carefully cut through and remove silicone-based caulk or softened PVC- or water-based caulk. Keep the blade at a low angle so you don't scratch the surface of the tub. You can also use needle-nose pliers to work chunks of caulk out of the seam.
Remove any remaining film using an alcohol-based product. Remember, you don't want to introduce water at this stage.
Use a mildew-killing solution, such as bleach, to remove any mildew in the area. Wipe it completely dry or wait several hours before re-caulking.