The chirp starts at night, once every minute or so, and it is the kind of sound that makes you want to pull the whole unit off the ceiling. Before anyone climbs up, read what the alarm is telling you. Many units chirp for a low battery, and many chirp when the unit has reached the end of its working life. The two need different fixes, so find out which one you have first.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Check the date before you touch the battery
Look on the back or side of each unit for a manufacturer date or an end-of-life date. If the unit is past that date, a new battery will not solve the problem and the whole alarm needs replacing. If the date is still ahead and the chirp continues, a battery is the likely fix. Write down what you find, including the date, so the next person does not have to guess.
You do not need to be in the house to start this. Ask whoever is there to read the labels to you by phone or video. Also ask what kind of unit it is: a battery-only unit, or a unit wired into the house power, sometimes with a battery backup. The answer decides whether a battery swap is an afternoon job or a job for a licensed electrician.
Do it yourself on a visit, or hire it out
Changing a battery on a stepladder is a reasonable job for a family member on a visit or a neighbor who is comfortable on a ladder. Replacing a whole unit, especially one wired into the house, is usually a job for a licensed electrician or a handyman who will take responsibility for the wiring. Before any hire starts, ask three things: whether they will test every unit in the house, whether they will replace the whole unit or only the battery, and who handles a unit that keeps chirping after the swap. A good hire answers all three plainly.
Testing is the easy part. Press the test button on each unit and listen for the full alarm. Ask the helper to test on every floor, including the hallway outside the bedrooms and the top of the stairs. Those are the places where the first warning has to be heard.
Carbon monoxide units belong on the same list
Many homes have a carbon monoxide alarm near the furnace, the water heater, or the garage, and some combine it with the smoke alarm. Check it the same way: find the date, press the test button, and confirm it is placed where the manufacturer's label says. Do not assume a unit is working because it has a light on. If you cannot tell whether the house has a carbon monoxide alarm at all, put that question to the furnace or water heater service person on their next routine visit.
Make a short list before anyone climbs
Write down how many alarms the house has, where each one sits, the date printed on each, the last time someone tested it, and who has access to the ladder and the key. Keep the list with the house papers so a helper or a hire can see it on the first visit. If you need someone near your parent's home to look at the alarms this week, the local help page for a St. Louis parent is a practical place to start. The sources page shows where the testing and placement guidance comes from, so you can check it against the label on the unit itself.
Tonight
Ask someone in the house to press the test button on each alarm and read the date on the back of each one. Write the results on one page. Then decide, for each unit, whether a battery, a whole new unit, or a licensed electrician is the right next step. Do not let the chirp make that decision for you at two in the morning.
What to leave with the hire: the list of alarms, their locations, their printed dates, the test results, and a clear note that the ladder and the key are arranged before the visit.