To replace a porch light fixture, you will need some supplies. You will need to get a sturdy ladder, a screwdriver, and possibly a hammer as well, if the fixture is old.
Before you begin, make sure that the power to the porch light is turned off. Place the ladder in a stable area but within reach of the light fixture. If you can, ask someone to help you by holding the ladder and staying nearby. This is because you are going to want to hand down the light fixture to someone so that you do not drop it and it does not break. Depending on the size of the fixture, you may be able to handle it by yourself.
Climb the ladder until you are within reach of the light fixture. Using your screwdriver, remove the screws carefully. If they are painted on, lightly tap on the fixture with a hammer to loosen the fixture from the paint. Hold the bottom of the fixture with one hand and loosen the screws with the other, so that the fixture does not suddenly drop from the porch. In this manner, remove the fixture. This will expose the electrical housing unit that the light bulb is attached to.
If the lightbulb is attached to the fixture and not the electrical housing, you will have to detach the electrical wires from the fixture. To do this, simply untwist the wires on the fixture from the wires on the housing.
To install the new light fixture, carefully lift it up into position and follow the installation directions for that particular fixture. Most light fixtures are installed the same way, which means they come with screws that are made to screw into the electrical housing. This means that you will most likely not have to drill any new holes, unless you have a special or unusual light fixture.
However, if the lightbulb is meant to screw into the light fixture and not the electrical housing unit, you will have to twist together the wires from the fixture to the electrical housing. Simply match the colors of the wires and twist them together. White goes to white, and black to black. Then, screw in your light fixture and turn the power back on to your porch.