I would use a heat gun to warm the adhesive and then use a plastic prying tool like found in a set of interior trim removal tools to gently pry the letters off. I just did this to replace the Ford emblem on my F-150's tailgate. Worked perfectly. Only had to warm up the left over adhesive to remove it from the painted surface. I have a

plastic razor blade scraper" target="_blank">plastic razor blade scraper

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for doing this type of stuff. Bought it off Amazon. Make sure to get one with the plastic razor blades if you're interested. Won't damage the painted surface.