These directions are for a single switch that controls one or a set of lights. If you're working with a three-way switch (where two switches control the same light) these will not apply.
Tools needed: Screwdriver, new light switch
Step One: Turn off the breaker or fuse to the switch in question. If you're not sure which breaker controls the switch, turn off the main to be safe.
Step Two: Remove the plate cover
Step Three: Unscrew the switch from the box.
Step Four: Pull out the switch and assess the situation. It should have two coated wires coming from it. If your switch is grounded, it will have a third wire, this ground wire is usually an uncoated copper wire.
Step Five: Remove the old switch from the wires. Depending on the type of switch the wires will be connected differently. Pay attention to how they're connected as you'll most likely want to connect the new wires the same way.
Step Six: If you have a ground wire, connect it to the new switch - it connects to the green screw at the bottom of the switch
Step Seven: Connect the wires to the switch. In a standard one way switch, the order of the wires doesn't matter. The wires will either connect by being pushed into two holes in the back of the switch, or being side mounted to the copper screws on the side of the switch. Make sure the connection is solid.
Step Eight: Screw the new switch into the box. If you're in an older house, the electrical box may be made of out metal. If this is the case, be very careful that no exposed wires (with the exception of the ground) are touching the metal box. Make sure it's right way up - when it's flipped down it will say "OFF" if it's the right way or "NO" if it's upside down.
Step Nine: Put the plate cover on
Step Ten: Turn the power back on and test your new switch. If it doesn't work, chances are you didn't make a good connection with one of the wires, go back to Step One and start again.
Enjoy your new switch!