Just rename the name off the .app in the Applications folder. I'm not positive if you can do it with the default apps or not. But I know you can do it with custom apps.
if you dont have it already download MobileFinder. Open it up and find the Settings button. Click that and scroll down till you see System File Access. Turn that on. Press the finder button. Go to the /Applications folder if you are not already there. Select the app you want to change. Only press it once. With it selected, press Modify, located at the bottom. In the field that says filename, delete everything but the .app and rename it to what you want press done and respring and that should be it.