You are replacing a smoke alarm. Start with the old one, because its back label tells you how long it was meant to last. Turn it over before you touch the new box. The label is the planning input for the whole job.
Read the end-of-life mark first
Most smoke alarms print a manufacture year or a replace-by mark on the back. Find it and note it. If the unit is past that mark, replacing it is the plan, whatever it looks like from the ceiling. Write the replace-by year on the new unit in permanent marker, on the edge that faces the wall, so the next person can read it without taking the unit down. Add the install year to your owner's file as well.
Choose the type and the power source
Check three things on the box before you buy or install. First, the sensor type. Some units use ionization sensing, some use photoelectric sensing, and some combine both. Ask the seller or read the spec panel to confirm which one you are getting. Second, the power source. A sealed battery unit runs on its own battery for its whole service life. A hardwired unit draws power from the house circuit and usually has a battery backup. Third, interconnection. Some units link wirelessly or through wiring so that when one sounds, all of them sound. Match the new unit to what the old one did, or upgrade on purpose and note the change.
Place it where it works
Put a unit inside each sleeping area and outside each sleeping area. Put one on every level of the home, including a basement. Keep the mounting spot away from kitchen steam, bathroom steam, and the direct airflow of a supply register or ceiling fan. Keep it out of the dead air of a corner. Follow the clearance from walls and corners printed in the manual. Ask yourself whether the spot still makes sense if the hallway door is closed, because that is how most people sleep.
Remove the old base and mount the new one
Twist or unlock the old unit from its base and set it aside. If the unit is battery only, remove the base screws, check the wall or ceiling for old holes, and fill them before you mount. Set the new base with the screws or adhesive the manufacturer specifies, and make sure it sits flat. Snap or twist the head onto the base until it locks. Give it a gentle tug to confirm it holds.
Wire a hardwired unit with the power off
If the unit is hardwired, shut off the breaker that feeds the circuit and label it so no one turns it back on during the job. Confirm the wires are dead with a voltage tester before you touch them. Match the wires by color and terminal to the new unit, following the diagram in the manual exactly. Tuck the wires back into the box so the cover sits flat. If you are not sure about any wire, or the box looks old, damaged, or undersized, stop and bring in a licensed electrician. That is the point where a professional should own the work.
Interconnect and test each unit
If the new units link to each other, pair or connect them according to the manual, then confirm the link works. Press the test button on each unit and listen. The alarm should be loud and clear, and every interconnected unit should sound together. Check the battery indicator on sealed or backup units. Press the hush or silence button once to confirm it does what the manual says. Test again after the cover is on, so you know the final setup works.
Log it and handle the old unit
Write the install year, the replace-by year, and the location of each unit in your owner's file. Put a short note next to the test button on the old unit's old location if you want a reminder of what was changed. Dispose of the old unit according to local rules. Some areas accept smoke alarms at household hazardous waste sites, and some ask that the battery come out first. Check before you carry it to the curb.
Questions to ask if someone else is doing the work
If an installer, property manager, or landlord is doing the swap, ask who supplies the unit, who confirms its replace-by year is recorded, and who tests it at the end. Ask whether the new units are interconnected and whether the work was done to the local code in your area. Ask who keeps the record. The answers will tell you whether the job is finished or just started.