Depending on what your HVAC (heating, ventilating and air conditioning) system does, it may have two, four, five, six or seven thermostat wires. Two-wire systems are heating only, four-wire systems are heating and air conditioning, five-wire systems are for thermostats that receive their power from the HVAC system, six-wire systems have two stages of heat and seven-wire systems have two stages of heating and cooling. The seven-wire system is most efficient because the HVAC unit always runs in the lowest stage first before kicking up to the higher output stage. While it looks challenging, it is really fairly easy to install a thermostat with seven wires.
Turn off the power to the HVAC unit by turning off the correct circuit breaker in the circuit breaker panel. Verify that the power is off by checking the power at the unit with a volt meter.
Take off the HVAC unit access panel and look to see how your thermostat wires are connected to your HVAC unit. For a seven-wire system, a typical configuration would be: the red wire will be on the "R" terminal, the white wire will be on the "W" terminal, the green wire will be on the "G" terminal, the yellow wire will be on the "Y" terminal, the black wire will be on the "C" terminal, the light blue wire will be on the "Y2" terminal and the brown wire will be on the "W2" terminal. Put the access panel back on.
Install the thermostat wall plate to the wall with drywall anchors and screws. Use your level to check that it's level before proceeding.
Connect the thermostat wires to the thermostat in the same fashion that they are connected to the HVAC unit. Strip them, if necessary, so 1/4 inch of bare wire is exposed on the end of the wires.
Install the thermostat to the wall plate. It should snap into place by gently pressing it on. Make sure the thermostat wire doesn't get pinched behind the thermostat when pushing it onto the wall plate. There should be a little depression where the wire is meant to sit.
Turn the power back on to the HVAC unit. Set the temperature to your desired setting, and the HVAC unit will maintain that temperature.