There isn't an access to the bulb itself as it is a sealed unit. You'll have to swap out the whole unit. This is easily done by pushing the 4" unit out and then disconnecting the plug/wire. It's an incandescent 4" seal unit and not expensive.
I elected to replace all four of mine with LED units which can be purchased in the automotive section of Wal-Mart. They again are 4" sealed LED Blazer p/n C542RTM. If you go this route with LEDs, you'll also have to replace the flasher unit under the dash. I went with Grote 13 Pin Electronic Lighting Control Module 44160.
The whole setup complete was around $165 with the flasher unit being the most expensive part. I believe each 4" LED sealed unit was around $15 each.
The results were amazing as the LEDs are much more noticeable and kind of updates the looks of the Cruiser.