by replacing an older manual thermostat with a modern more accurate programmable one you'll rid your house of dangerous mercury and save on energy costs first locate and turn off the emergency furnace switch next remove the cover of the thermostat remove the screws that hold the thermostat to the base plate the thermostat will lift off the base look at the thermostat wiring there will be either two or four wires depending on whether the thermostat is controlling just the heat or heating and cooling unscrew each wire from the base plate these 24 volt low-voltage lines connect to your furnace the voltage is too low to harm you using a screwdriver remove the screws fastening the base plate to the wall and gently slide the wires through the hole in the base plate as you lift the base away from the wall as you remove the thermostat hang on to the wires temporarily tape the wires to the wall so that they don't slip back into the wall while you're mounting the new thermostat inevitably you'll have to drill new holes for the base of your new thermostat using the base plate of the new thermostat as a guide carefully mark the locations for the new holes for the wall anchors pick a drill bit that's the same size as the wall anchors that come with the new thermostat and then drill a hole for each one of the wall anchors and then press or tap the new wall anchors into the holes align the base plate of the new thermostat over the wall anchors and secure it in place with the screws provided now following the directions that came with the new thermostat connect the wires to the proper terminals depending on your new thermostat insert the batteries these batteries should be replaced once a year some thermostats don't have batteries but run off the low-voltage power supplied by the furnace now attach the new thermostat to the base with the thermostat installed you can now program their new thermostat to change to different temperatures throughout the day and into the night this allows you to set the temperature lower when your family is typically out of the house or sleep thus lowering your energy costs