I always try to scrape as much as I can off with something that doesn't gouge the surface, ie plastic scraper, old credit card, or even my finger nail if it's relatively thin.
Anything metal or hard will scratch the surface and leave a "mark"
After you get most of it scraped off, use some regular old brake cleaner on a rag corner to wipe it all perfectly clean. Brake clean does two things, breaks up silicone, and prepares the surface free of any residual oils/dirt for a fresh new coat of nicely beaded silicone.
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