Argh! The word "knife" anywhere near the screen makes me shudder!

You need to unscrew the two screws inside the top, and then use the gap where the screw used to be next to the headphone port to put in a small screwdriver and lever the top-left corner of the screen out. The sim-slot-side screw levering is harder because there's nothing solid to lever against and you need to try to not damage the case (if you want to keep it).
Once enough of the top is away from the case, use a guitar plectrum or similarly solid small plastic thing to prise the screen away all the way round.
There's videos of this being done, linked to on this thread I think.