Changing a three-way switch to a two-way switch is generally part of a remodel of a room. A three-way switch is single pole double throw (SPDT) with three connection wires, and is generally wired with 12/3 or 14/3 cable between the two switches. Two-way switches are single pole single throw (SPST) with two connection wires, and are wired with 12/2 or 14/2 cable between the switch and fixture. Changing one of the three-way switches to a two-way switch will require the deactivation and removal of the second three-way switch.
Start with the three-way switch that will be removed. Remove the switch cover plate screws with the slot screw driver, and then remove the two switch screws. Carefully pull out the switch, making sure not to touch or short any of the three wires. Locate the common lug on the three-way switch. The common is generally a darker colored screw and is marked as common on the back of the switch.
Turn off the power to the circuit at the breaker box. With the voltmeter, verify that the power is off. Disconnect the three connections from the switch. Straighten the wires out as necessary, and strip insulation to about 1/2 inch. Connect together the wire from the common lug and one of the traveler wires. Use the lineman's pliers to make a strong mechanical connection by twisting the wires together, and then twist on a yellow wire nut. Twist on just a yellow wire nut on the leftover wire. Neatly tuck all the wires into the box, and install a blank cover plate.
Move to the other three-way switch and remove the cover plate and switch screws. Turn the power back on at the breaker box. Flip the switch to make sure the light turns off and on. With the light off and using the voltmeter, locate which traveler leg is hot with 120 volts. Turn off the power at the breaker box. Disconnect the three wires from the three-way switch. Connect the wire from the common lug to the switch leg of the two-way switch, and connect the traveler leg that was hot from the test above to the power leg of the two-way switch.
Carefully, tuck the wires into the box. Reinstall the switch screws and cover plate. Turn on the power at the breaker box and test the light.