I've worked in body shops doing paint and body work so I have often removed decals. I use a heat *** (hair dryer will work) to soften adhesive. Then using a plastic scraper I pull up an edge and peel it off. Then I use acetone (90% rubbing alcohal may work depending on adhesive) to remove adhesive. Saturate a section of rag with acetone and hold it against adhesive for a minute or two and it should rub off pretty easily.