To change shelf material to glass,
Select the desired shelf.
Click on the Panel tab in the left panel.
Select the Materials drop down menu and specify the Material Source to be 'Self' in the corresponding dropdown.
Navigate to the Core Material header and select Click to Change.
In the carousel, navigate to Glass Ply and select the desired glass ply by clicking on the thumbnail.
The shelf panel will now have glass loaded into all its layers automatically.