Taking it back under warranty would be best.
If you can't then get some compressed air and try to clean it out.
if that doesn't work you probably need to replace keyboard.
get one from eBay - make sure it's new, matches your model and identify it by look.
As above, you should find a YouTube vid explaining how to fit it - normally it will be screws underneath marked with keyboard symbol, then possibly bezel off, then prize out keyboard - hardest thing is normally those last two bits, and you really need the right tools otherwise it's very easy to damage the plastics.