I used the small triangle blade on my exacto kit( del,l actually it's my wife's and she hides her tools from me, lol) but I to lift a corner from the new decal sheet and apply it to the cleaned off area. On my Addams, I used the flat exacto blade that is rectangle in shape to remove the old decal, cleaned it up with a little Goo Gone and some novus2 and then applied the new decal.
Just did the same on a Fire I'm cleaning up to sell the other day.