Before you touch anything

Find the manufacture date on each alarmSmoke alarms need to be replaced 10 years from the manufacture date, not from when you moved in or installed it.
Check CO alarm age tooCO alarms don't last forever. Replace CO alarms every five to 10 years depending on manufacturer. Check the manufacture date on the back of the alarm, as well as the owner's guide for replacement recommendations.
Note which alarms have a sealed, non-replaceable 10-year battery vs. a regular swappable oneYou'll treat them differently below.

Test and replace batteries

Press the test button on every smoke alarmTest smoke alarms once a month using the test button — the clock change is just your cue to do it today, and to swap the battery while you're at it.
Replace batteries in standard-battery alarmsSmoke alarms with any other type of battery need a new battery at least once a year — now is that time.
Leave sealed 10-year-battery alarms aloneSmoke alarms with non-replaceable 10-year batteries are designed to remain effective for up to 10 years. If the alarm chirps, warning that the battery is low, replace the entire smoke alarm right away.
Never repurpose a smoke-alarm battery for something elseA remote or toy isn't worth an alarm going dead without you noticing.
Test every CO alarm the same wayPress and hold the test button on each one and confirm it sounds.
Replace CO alarm batteriesOn the same once-a-year schedule as regular smoke alarms, unless it's a sealed long-life unit.
Dust or vacuum each alarm gently while it's open for the batteryA dusty sensor is a common cause of false alarms and missed real ones.

Replace the whole unit if any smoke alarm is 10+ years oldReplace the entire alarm if it is more than 10 years old or does not work properly when tested — no fresh battery fixes an expired sensor.
Replace any CO alarm past its stamped expiryAn old CO sensor can read normal while quietly no longer catching a real leak, so age matters even if the test button still beeps.
Replace instantly if an alarm failed its test, won't stop chirping on a fresh battery, or looks damagedDon't wait for a "better time."

Confirm there's a smoke alarm in every bedroomPut smoke alarms inside and outside each bedroom and sleeping area. A closed door may slow the spread of smoke, heat and fire.
Confirm there's one outside every sleeping area and on every level, including the basementSame reasoning as above.
Confirm at least one CO alarm is near the sleeping areaIf only one CO alarm is being installed, the Consumer Product Safety Commission (CPSC) recommends it be located near the sleeping area, where it can wake someone sleeping.
Add a CO alarm on the level with a basement, attached garage, or fuel-burning appliance if has basement garage or fuel applianceGas furnace, water heater, fireplace, etc.
Check for a bed-shaker or strobe alert device paired with the alarms if household member deaf or hard of hearingA standard beep won't wake someone who is deaf or hard of hearing.
Link interconnected alarms and test that they all sound together if has interconnected alarm systemOne going off should trigger every unit in the house.

Write the test date on a sticky note or your phone calendarSo you're not relying on memory until the clocks change again.
Walk the house once to confirm every alarm you expect is actually there and lit upIt's common to find one missing or painted over during a renovation.
Make sure everyone in the house knows what the alarm sound means and where to goA quick reminder costs nothing and matters if it ever actually goes off.