I just overlay the new one over the old one. When the stack gets to be too noticeable it is time for a new boat. Or, you could use a hair dryer. Don't get too close, just warm it up and it should come off. A little lacquer thinner should remove any goo that is left.
There are a number of solvents to remove "gum"...such as "goof off", "Oops" etc. Also a heat gun will work well--use it gently. But I agree, usually I just put the new sticker on top of the new.
Do not use a putty knife. If you need to scrape, then use a plastic scraper, such as used for feathering Bondo or other fillers.
