The first thing to do is mechanically remove the silicone caulking using a knife or scraper. You should be able to remove most of it this way. Use a silicone caulking stripper to remove the rest. Be very careful using the stripper, as it may remove some of the paint on the aluminum siding. You must use caulking stripper, however, because it is essential that all of the caulking is removed-silicone caulking is very popular because nothing will stick to it, including more of it. If the caulking you want to remove is not removed completely, the new silicone caulking will not hold and you will have leaks. It is for this reason that I always recommend polyurethane caulking. As a sealant, it works as well as, if not better than, silicone caulking, and if you need to retouch the job, the new caulking will adhere to the old.