Remove the base of the fixture or other area to access the wiring compartment or switch. Unplug (or otherwise disconnect) the fixture from the power source. Shutting it off with a wall switch is NOT considered "disconnected". Accessing the wiring compartment depends on the fixture. Some require a plate, screws, or other protective barrier to be removed. Sometimes it may be a simple piece of cardboard glued to the bottom of the fixture base. Try to remove it causing a minimal amount of damage, as it will be needed to be reinstalled when done. Under the base cover (cardboard) you will find one or more of the following: transformer, power cord and the control switch (unless you are working with a pole lamp Torchierre type where the on/off/dimmer switch is in the pole).