This is a really, really difficult fix. You can use a craft knife to pick out the glass off the housing. Bear in mind that the glass is very sticky and will be difficult to remove. A suggestion I've hear a couple of times is to use a paint-stripping gun (you know those things with a heated element at the end) to see if that would just lift the glass off. You'll then need to use double sided tape to glue the new glass on.
Good luck! 🙂