For starters, install your programmable thermostat unit on an interior wall, away from heating or cooling vents and other sources of heat or drafts (doorways, windows, skylights, direct sunlight or bright lamps).
Remember: Read all instructions and proceed carefully! Programmable thermostats are a low voltage wiring installation and involve anywhere from 2–10 wires, depending on your type of heating and cooling system. However, you should shut down your electricity during any replacement. The previous attachment points will reconnect your new unit.
If the job requires more than just a replacement, call your certified HVAC professional to ensure proper installation, as well as operation of your heating and cooling system. It's a good idea to upgrade an old manual thermostat to a programmable unit if you're replacing a CAC or heating system given that programmable thermostats are far more accurate and will maximize the efficiency of your new system. Heat pumps may require a special unit to maximize energy savings year-round. Talk to your retailer or HVAC contractor before selecting the thermostat.
Also, if you're replacing a manual thermostat that has a mercury switch, be careful not to break the tube that holds this toxic substance. Contact your local recycling/hazardous materials center, or the manufacturer of your new thermostat, for advice on proper disposal.