Blow dryer to soften the adhesive and...
patience enough to hold the blow dryer on it till it does. A heat gun is faster, but unless you really know what you're doing with a heat gun, I don't recommend it...'cuz only want to soften the adhesive, not melt the plastic.After you get the letters off, remove the adhesive residue with a liberal application of WD-40 ...have a beer while you wait for it to work...then scrub with a plastic scrubber. Getting it all off may require two beers. Then wash well with detergent and water to clean up the WD-40.