First, turn off the breaker supplying power to the UV light, then remove your furnace's lowest panel. The ultra-violet light for your AC system is located in the bottom of the furnace and should be removed and replaced.
Behind the panel is an A-coil, which looks like an inverted V. Inside the A-coil is a second panel, normally triangular in shape, which also needs to be removed. With this second panel removed, the ultra-violet light and its wire plug are evident. Unplug the light.
The light fixture is removed by handling the base fixture of the light, not the bulb. The bulb is fragile and will break. There are two basic methods for attaching the UV base fixture, magnetic or with screws. If the fixture is magnetic, pull the base fixture away from the furnace wall it is attached to. If it is attached with screws, unscrew the base. The new base fixture can now be attached, using the magnet or the screws as applicable.
Once attached, plug the fixture in, and test it by turning the breaker back on. Without staring at the UV light, determine if the fixture is working; if not, the problem may be the UV system's transformer. If it is working, turn the breaker back off, and begin the close up process. Replace both the A-coil panel and the exterior panel.