To install a Honeywell thermostat, shut off the electricity to the old thermostat, label the wires, remove the old unit, and then connect the wires to the matching terminals on the new thermostat. Visit Honeywell's website or check the manual for more specific instructions, which vary slightly by model.
At the main electrical panel, shut off the power to the thermostat. Use a voltage sniffer to test the wires for an electrical current. Once the power is off, remove the faceplate or display cover from the old thermostat. Label the wires with their terminal location. Disconnect the wires, and tape them to the wall to avoid losing them. Unscrew and remove the old base or wall plate.
If the new thermostat has a separate base, mount it on the wall with the wires threaded through the opening in the center. Following the labels, connect the wires to the appropriate terminals on the new thermostat. Push any excess wire length back into the junction box.
If needed, insert a battery into the thermostat, and then attach the thermostat to the base. Once the new thermostat is installed, turn on the power at the breaker panel, and the thermostat is ready to be programmed. Depending on the model, programming options vary, and specific instructions are available in the owner's manual.