Use a exacto knife razor blade to get underneath them and they should come right off. Then, use a washcloth (do not use a paper towel or other paper product, they leave lint and parts) with rubbing alcohol to remove the left over glue that will remain. Feel the surface with your hand as they will be little "glue balls" left that you can pick off. You can also use a product call Goo B Gone that you can buy and any hardware store or drug store, but you will have to follow up with rubbing alcohol since it leaves an oil film behind and the replacement numbers won't stick. However, Go B Gone really works fast and without any elbow grease. This process should remove it all so you won't end up with unsightly shadow number as dirt gathers on the old glue.
Being former military and moving around a lot and having to re-register my boats (since Soldier and Sailor's Relief Act doesn't apply to vessels, only cars), I got pretty good at this procedure.