Replying to Detuning my edges?

Well, I just got new boots, and my parents make me race, so I dropped off my invaders and my race skis at the local ski shop to get the bindings set to the new boots, and told them to tun up my race edges, but NOT my invaders. Anyways, they decided it would be a good idea to just tune both, and now, next time I hit a rail on my invaders, I can look forward to smashing my face when my now sharp edges catch. So the question: How would I go about detuning my invaders' edges? how do I know how much to take off, what tool to use, what angle to go at? I don't want to take too much, since I use them outside the park, but enough that they won't catch on everything. Thanks.