The method for replacing a broken taxi roof light depends on the style of light and the degree of damage. In most cases, the plastic outer casing needs to be removed, the bulb unscrewed and replaced with a functioning bulb that meets the wattage requirements.
If the taxi roof light bulb is burnt out then the method for replacing it simply involves taking it out and installing a new bulb. The car should not be running during the replacement, and the light should not be in the on position. This method doesn't require the light casing to be removed from the car. If the bulb still doesn't turn on with a new bulb, or if it quickly burns out, this is a sign of a larger electrical issues and means the full fixture needs to be removed and the wiring inspected.
If the light bulb is cracked or broken, replacing the bulb requires a higher level of care. The person performing the replacement must wear work gloves for protection and make sure that there is no electricity feeding into the light. Once the power is off to the light it is removed by either gently turning the metal fixture or by placing a cut potato over the broken bulb to work it loose.