The clear acrylic top and bottom of the phone has been lasercut. It’s got a sharp edge all around the outside. You can tell it’s there because a fingernail will catch on the edge of the plastic. It’s very thin and thus very sharp. You could cut yourself on it!
Break the sharp edges by scraping a knife perpendicular to the edge, to wear the sharp edge down. You know you’ve taken off enough plastic when it doesn’t catch your fingernail anymore. You could use a bit of abrasive like sandpaper or emery paper, but the knife works fine.