Using the sharp implement, VERY carefully move around the edge of the glass screen and prise the screen up out of the case.

You'll probably need to go around the edge of the Garmin a few times before it comes totally free

There is double sided adhesive holding the screen to the case, and you just need to break that seal to get the screen out

Be VERY careful you don't push your sharp implement too far into the device, as there is a delicate connector near the bottom right hand corner that you could damage

Once you have broken the seal all the way around the perimeter, the screen should just lift off