Unless it's real complicated, do it yourself.
As King stated use a hairdryer and razor blade to lift the old decal off the boat and then clean the area. If the old lettering is stubbornly attached, spray with easy off oven cleaner and let it sit ofr a few minutes. Then clean the area prior to attaching the new lettering.
When attaching the new, have a spray bottle of soapy water and keep the area wet. This will allow you to slide the lettering around until you have it where you want it. Then use a small plastic scrapper, or credit card and work out all of the air bubbles starting in the center and working towards the edges. If it starts to pull at all, give it another spray from the spray bottle and it will glide again.
Stay patient, as you have a lot of time to work with it as long as you keep it wet.