Shut off power to the ceiling fan at the household breaker panel to prevent it from accidentally turning on while you change the diffuser.
Loosen the screws around the rim of the existing glass diffuser but do not remove them. Hold the diffuser with one hand while loosening the screws to keep it from falling.
Measure the diameter of the diffuser to verify that the replacement will fit.
Hold the new diffuser up around the light bulb with one hand.
Tighten the screws opposite from each other in an X pattern to ensure even distribution of force to keep the glass from cracking.
Restore electricity to the light at the breaker panel then turn on the light to test its operation.