It depends on the model, but specific instructions are provided in your owner's manual.

On some models, like the TriloG™, IntelliG™, Excelerator®, and ReliaG™ 800 models: 

1. Simply depress the two tabs that connect the lens cover to the top of the unit (chassis); the lens cover will swing open.
2. Replace the bulb(s) with a service light bulb (follow guidelines in your owner’s manual for maximum wattage), and close the lens cover.

On other Genie® models, you will need a Phillips screwdriver.

1. Remove the two screws (at the top) that attach the lens cover to the front panel.
2. The lens cover will drop down like a hinge, while you change the bulb.
3. Use a heavy-duty, rough service light bulb (follow guidelines in your owner's manual for maximum wattage).
4. Realign the lens cover, and reattach the screws.