While it's possible to replace a single glass block if one breaks, it's a task that requires caution and expertise. The process involves carefully removing the damaged block by chipping out the surrounding mortar, which can be hazardous due to sharp glass fragments. For your safety, it's recommended to wear appropriate protective gear, such as safety goggles, gloves, and long-sleeved clothing. However, to ensure the replacement is done safely and correctly, it's best to hire a professional who has the necessary skills and equipment.