Initially, confirm any adjacent outsides are clean and dry. On heretofore caulked surfaces, it is critical that you remove as much of the old caulk as possible before applying new caulk. If the old caulk is hard feeling it is perhaps a water based caulk and to remove it you are most likely going to use needle nosed pliers. If the old caulk feels like rubber then it is a silicone based caulk and you will most likely need to use a razor blade to remove it.
