Turn off the power to the light fixture at the main service panel.
Remove the diffuser to expose the fluorescent tube(s).
Remove the tube by rotating it. Twist and turn the tube in any direction to disengage the tube and gently slide it out of the sockets.
Inspect the pins to see if they are bent or broken; if so, replace the tube.
The ends of a worn-out tube are dark gray.
Insert the new tube so that the pins slide fully into the sockets, then twist and turn in any direction until the tube is securely in place.
Reattach the diffuser and switch the power back on.