Bumper stickers and dealer emblems are usually attached to a car with glue. The best way to loosen glue is heat, so try heating it up with a hair dryer. Use a putty knife, with its blade wrapped in duct tape to work the label up as the glue warms up.
Don’t set your thermostat at a colder setting than normal when you turn on your air conditioner. It will not cool your home any faster and could result in excessive cooling and, therefore, unnecessary expense.