This article from Star-Telegram.com has the answer:
How long does it take to change a light bulb on the Cowboys stadium scoreboards?
Technically, there are no light bulbs on the scoreboards, but there are light-emitting diodes, or LEDs, that sometimes go dark. Workers can access the back of each scoreboard from an interior catwalk on the metal frame.
Each panel is made up of smaller LED "bricks," and each brick can be removed in less than 15 minutes, Mitsubishi’s Mark Foster said. Those bricks can then be returned to Mitsubishi’s Pennsylvania factory to be repaired. (Read the original article.)