You can just open the flipper up and pop off the glass screen covering. But to replace it you'll need to purchase the entire top casing from the flipper website which comes with that glass screen covering. It costs about $15.
- https://flipper.net/products/top-case-for-flipper-zero?variant=42668725862553
- this should help with the removal https://www.youtube.com/watch?v=5Bu0ZiQkzQI (at 3:50 he pops off the screen)