It's very easy to replace a panel in an exoterra terrarium, for starters the panels don't normally butt up tight together so it's easy to slide a blade down between them and cut the silicone, the black silicone also doesn't stick to the black plastic very well so the plastic trims and glass can be parted easily. They use 6mm glass (or at least the 45x45x60 and larger do, not sure about the smaller sizes) which should be available cut to size from a local glazier then just silicone it all back together using aquarium safe silicone.