How do I replace the pull chain for the light on a ceiling fan?

Buy the switch at Home Depot.
Disconnect electricity to the fan by circuit breaker or wall switch.
Remove all light bulbs and glasses tulips from bulbs housings
Take out the 3 screws holding the fixture to the fan disconnect the 2 wires (usually white and blue) and replace the switch comfortably on your desk.

VERY simple, here are instructions:

Ceiling fan pull chain replacement and repair - Ceiling Fans N More