Turn off the power to the house to prevent electric shock. Use the flashlight or lantern to light your work area.
Remove the old thermostat from the wall. In most cases, this means pulling the cover off to reveal the screwed-on base and unscrewing the base from the wall. This will expose the wiring behind the old model. Leave the wires temporarily attached to the old thermostat.
Label each of the wires connected to the old thermostat based upon the letter of the terminal in which they were screwed.
Unscrew the labeled wires from the terminals and strip the ends so that one-quarter inch of wire is exposed. Wrap these wires around a pencil to prevent them from being lost inside the wall.
Pull the cover from the Honeywell thermostat. Loosen the screw at the bottom that locks the unit to its base, and lift up the thermostat unit to access the base behind it.
Put the plastic anchors that came with the Honeywell thermostat into the holes from the old unit.
Unwind the wires from the pencil and push them through the large square hole in the base of the unit. This will allow you to wire the new thermostat. Push the base against the wall and insert the screws. Screw them into the anchors in the wall.
Attach the wires to the terminals of your new thermostat in the same way that they were wired to the old unit. For diagrams to wire various possible setups, refer to the manual in the resource link section. In most cases: Rh is the heating power supply; Rc is the cooling power supply; W is the heating signal; Y is the cooling signal and G is for the fan.
Look on the back of the electronic unit for the thermostat. Find the J2 jumper switch and set it to HE if you have an electric furnace, or HG if you have a gas furnace. This will adjust the fan settings for how your home is heated.
Pull the battery cover off of the front of the electronic base. It will be located just above the display screen. Insert the 2 AA batteries and replace the cover. The display should begin to run several tests. When those are finished, it will show the current temperature.
Reattach the electronic thermostat to the base by snapping the top two corners on first and tilting the unit down. Reset the locking screw to prevent the electronic base from moving.
Snap the faceplate back over the electronic unit, and turn the power back on to the house.
Refer to the instruction manual for this model in the link section for programing instructions for your Honeywell RTH230B programmable thermostat.