Silicone caulking is used to seal interior and exterior joints against moisture. Over time, silicone caulking breaks down and must be replaced. However, if you try to simply smear new caulking on top of a pre-existing layer, the result will be a poor bond and a failed job. You must first remove the old layer of silicone caulking before you can add the fresh material.
Scrape away the bulk of the old caulking with a five-in-one tool, available at any hardware store. You can also use a stiff putty knife or even a razor blade if you have very steady hands. Whichever tool you use, you must be careful not to damage the surrounding area. Water works well as a lubricant to keep the tool from gouging into the surrounding area.
Apply a silicone caulk solvent, either store bought or homemade, to any remaining silicone that will not scrape off. You can make your own solvent by mixing strong powdered cleanser and water to form a paste.
Allow the solvent to soften the remaining silicone. The time required will depend on the amount of silicone caulking and the strength of the solvent.
Scrape away the softened silicone caulking with the removal tool.
Wash off any remaining solvent and allow the area to dry for 24 hours before adding new caulking.