Can you change a 94 Chevy G20 van headlight bulb or is it fixed unit?

The 1994 Chevy G20 van's headlights are replaceable bulb units, not fixed units. You can change the bulbs yourself. However, the exact procedure might vary slightly depending on the trim level and specific headlight configuration (e.g., sealed beam vs. composite). You'll likely need to access the back of the headlight assembly from the engine bay or through a small access panel in the front of the engine compartment. You'll need to consult a repair manual or online resources with specific diagrams for your van's year and model to be certain of the steps.