The six-sided slot is for a hex-head fitting. An Alan wrench or key is needed, and are commonly found at home improvement stores, etc. The picture on the left looks to be a special tool needed, so that one is probably best to avoid. Be sure to support the glass and put a folded towel over any metal or hard surfaces it might come into contact with. Tempered glass is tough, but when it breaks it does so in spectacular fashion into a thousand tiny pieces.