Hi Judy: Here are a couple of sites that may help:
Hi Judy. I'm Cindy. Use scissors to carefully cut the burn part of the carpet out. Then snip away some carpet fibers near a wall or inside a closet where it won't be noticeable. Glue these fibers onto the carpet to conceal the burn spot.
Hi Judy, I have done this method in my husbands truck where he has dropped cigarettes
Cut the spot out all the burnt pieces with a scissors then get a few pieces of the carpet cut it up and glue to the spot but some times you could just leave it once you cut the burnt pieces off