Fairly trivial. Remove old thermostat from wall, unhook wires (don't worry, they're low voltage -- but it is preferable to turn off the system first), follow instructions to install programmable thermostat and attach wires to it, set it for the type of heating system it's controlling (usually a switch on the back) install batteries, follow instructions to program. Turn system back on. Remember to replace batteries every year (I do it on my birthday, along with all the smoke detectors, flashlights, and other low-power or infrequently-used devices). It's a bit more complicated if you have forced hot air and/or central cooling; then you have a few more wires to keep track of and make sure they're connected to the right terminal. But it's still straightforward. If you don't already have a thermostat -- if you're trying to retrofit electric baseboards -- that gets ugly; you need to have the thermostat control a relay.