Trying to remove old sealant is likely to be a very slow and difficult process. That should only be necessary if the old sealant is dry and cracked. If the manufacturer used dicor or a similar sealant odds are very good that little or none of the original sealant needs to be removed. If the sealant is in relatively good shape, I would wash and clean it thoroughly with a final scrub using mineral spirits. Then you can apply another layer of dicor.
I have never used tools for applying dicor. On the roof I just get it reasonably smooth or use self leveling dicor. On the sides, trim, around windows and other areas that show, I have used blue tape, apply with a caulking gun and then after a few minutes smooth with a finger wet with mineral spirits.