For efficiency, location is key

It's not difficult to replace an old thermostat.
Where a thermostat is in your home is important. Ideally, it should be in an area where the most people spend the most time. Also, it should be about five feet off the ground and at least 1.5 feet away from an outside wall.
A thermostat should NOT be exposed to direct sunlight and should not be near other heat sources such as heater vents, windows, hot-water pipes or other appliances.
Also, don't put a thermostat near a stairway or in the corner of a room. A stairway can be exposed to a great deal of circulation, the corner of a room to very little. Each can provide atypical readings to the sensors in a thermostat.
Replacing the old
You can have an electrician install a new, programmable thermostat, or you can replace an old thermostat yourself by following a few simple steps:
UTurn off the heating/cooling system.
UTurn off the power to that system on your main electrical panel.
URemove the cover of your thermostat. (Often this is as easy as just pulling it firmly toward you, but you may need a screwdriver for one or two screws.)
UMark the wires from the wall (positive, negative, ground). Scribble their designations on small pieces of tape, if need be, and attach each tape to its corresponding wire. Chances are these are going to be color coded, but even colors will fade over long periods of time.)
URemove the old thermostat base. There will probably be one or two screws to loosen.
UIf you haven't already, read the instructions that come with your new thermostat.
UOn the new thermostat, there should be a base plate which you can then attach (probably with a couple of screws) to the wall.
UConnect the wires from the new thermostat to the wires from the wall per instructions.
At this point, it may be necessary to make one or two small adjustments to the new thermostat, depending on the type of heating system you have.
UIf battery backup is provided, now is the time to affix the batteries.
UAttach the new thermostat's cover plate. Often, this is only a matter of snapping it into place.
UAt your main electrical panel, turn on the power to the heating/cooling system.
USet your temperatures, times and days.