Ouch! I believe the screen on the Fitbit is version 3 of Corning's Gorilla Glass. You will have to decide if any of the methods to repair the scratches are worth the possibility of making it worse but a couple of things you can try without harm is a Magic Eraser.. just rub continually and see if it helps. also a white pencil eraser and rub it slowly and steadily across the surface.
some people claim toothpaste, corn starch and all kinds of stuff.. the 2 I mentioned are fairly common and accepted to work.
Lastly, it would be a drastic measure but you can sand and re polish just like a faded headlight but I would really weight out this option. Best of luck!
