How do I remove a hard-wired smoke detector. Please.
UnelectrocutedsofarMike

Should be able to turn counterclockwise, some colckwise. Pull down, unplug 3 conductor from back of detector, If Part of an Alarm or Fire System, it may have 2 or 4 wires, Removing should not be done and may dial, or sound trouble. New battery and installation will be reverse of removal.
Some may need small tool to release if lock option enabled. It is only plastic, light pressure.

Maybe I was unclear in my question. There is a hard-wired smoke detector on the ceiling. I want to remove it. How should I do it? Should I turn off the main power switch? Do I put plastic caps on the leftover wires. How many wires might there be?

You can remove power, remove detector, and disconnect 2 or 3 wires from detector, cap off, turn power back on. Usually smoke detectors are installed, not removed, Don't leave yourself or family unprotected. Take Care

We have +firex smoke detectors and they just went all off at he same time, They are battery back up and we checked for smoke and checked the carbon monixde dect and it was fine. What caused this?
?

They are probably photoelectric, so cooking and dust can set them off. Detectors should be cleaned sin some environments with a can of air. Some systems are wired that when one detector goes off, all go off.

They are interconnected, The one that sensed a problem may show red Led to indicate which one.