If its not clearcoated over, try Walthers solvaset or Microscale's equivalent. Eventually the decals will let go, but you still may be left with an outline or impression of where they were. Some cleanup might be needed.
If its clearcoated, you're going to need to get through the clearcoat first. Its hard to do it and only get the decals, but it can be done if you're careful. These alcohol wipes theyve been selling for sanitizing actually seem to work pretty well for light stripping. Go easy, as it might get the paint too. You can also try wet sanding with 1500 or 2000 grit sand paper.