You are standing in the hallway at the thermostat, and it says sixty-one degrees, and Dad says it is fine, and you can see him pulling the cardigan tighter. Sometimes the fix is not an argument. Sometimes it is a better thermostat, one with big numbers he can read from his chair and a schedule that runs without him touching it. Mom probably turned that dial for thirty years and never told you it was hard to read. This is a job you can do in an afternoon if you go slowly.
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Before you buy anything
Take the cover off the old thermostat and look before you order anything. Photograph the wires where they sit in the terminals, with the letters visible on the screws. You will mostly see R, W, G, Y, and C, and sometimes a few others. Count how many wires are connected and write that number on your phone. A thermostat that needs a C wire, the common wire that powers it, will not work on a wall that has only four wires unless you add one or choose a model that runs on batteries. Check that with the model's own page before you pay for it.
Next, find out what kind of heat the house has. A furnace, a heat pump, and a system with an air conditioner each want a different thermostat setup. A heat pump in particular has its own terminal and its own settings. Read the new model's compatibility list against what is actually in the basement or the closet, not against what you remember. Look at the buttons and the display in the store or the photos, too. You want plain labels, a display he can read without his glasses, and a setting that can lock the controls so a bumped button does not change the house.
The swap, start to finish
Turn off the furnace switch near the unit, and turn off the breaker for it as well. Do not trust the thermostat's screen alone. Then take a strip of masking tape and a marker, and label each wire with the letter from the old thermostat before you disconnect anything. Keep the wires from touching each other or the wall. Loosen each screw, slide the wire out, and tape it to a sticky note with its letter.
Remove the old base plate and mount the new one. Use a small level so it sits straight, because a crooked thermostat looks wrong for years. Feed each wire through the opening and put it into the terminal with the matching letter. Tighten each screw snugly, then give each wire a gentle tug to make sure it holds. Push any extra wire back into the wall so it cannot drop inside, and seal the opening if the old installation left a gap. Put in batteries if the model needs them, snap on the faceplate, and only then turn the breaker and the furnace switch back on.
The first test
Set the date and time, then set a simple schedule that matches how Dad actually lives. If he is up at seven and in bed at ten, a schedule with two or three changes a day is enough. Do not build a complicated program he will never use.
Test the heat first. Set it a few degrees above the room temperature and wait. Within a few minutes you should hear the furnace start and feel warm air at a register in the room. Then check that the fan runs when it should. If the house has air conditioning, test cooling the same way, but only if the weather allows it. Set the temperature to something comfortable for him, then lock the settings if the model has a lock. Leave a short note near the thermostat with the number he should keep, the name of the lock code if there is one, and your phone number.
Keep the old thermostat in its box with the manual and the wire letters written on the inside of the lid. You may need it someday, and it gives him something to say when he says the new one is too complicated. He will say that. That is fine. Let him say it, and tell him the cold morning will be the only test he needs.
When to stop and call a licensed heating technician
Stop the job if you see any sign of a gas smell, sparks, scorched wire, or a burning odor. If you smell gas, leave the house, then call the gas company from outside. If the wires do not match the letters in a way you can explain, or if the old thermostat ran on higher voltage through a line you cannot identify, call a licensed heating and cooling technician before you connect anything. If the heat does not come on during the test, turn the breaker off and have the furnace checked before you try again. The thermostat is the easy part. The furnace is the part that needs someone who works on it every week.