Turn off the power to the furnace. If the switch is nearby and easily accessible, use it or turn the power off at the circuit breaker. Always verify that the power is off with a multimeter because switches and circuit breakers can be mislabelled.
Read the thermostat instructions and remove the old thermostat, being very careful not to let the wires fall back into the wall. If the furnace and air conditioning have already been installed and you are only replacing the thermostat, then the wire count should be correct. If this is part of a new appliance installation, check the number of wires running to the thermostat. If the wire count is incorrect, replace the wires with the correct number or hire a professional to do it.
Read the instructions included with the furnace and the wiring diagram that can be found on the inside of the furnace door.
Line the wire colors with the appropriate slots in both the thermostat and the furnace. Typical placement includes the red wire joining with RH with a jumper to the RC. Yellow is connected to Y, green attaches to G and the white wire joins to W. Two-stage heating and cooling requires additional wires and may not have standard colors. A wire will need to be attached to W2 for the second-stage heating and one attached to Y2 for second-stage cooling. Note what color is used and attach the same color to the color-letter combination on the furnace circuit board. A common wire will more than likely be needed and is usually color-coded black. The common wire connects to the C in the furnace and thermostat.
Place the thermostat faceplate back on the thermostat and secure the thermostat to the wall.
Inspect the wiring in the furnace to make sure that no wire can come loose with a little pull and all wires are secured in the correct location. Place the furnace door back on.
Turn on the power and program the thermostat using the instructions that came with the new thermostat.