Did this myself, as noted above there is a contractor package, Lowes was 6 of the smoke detectors for the price of 4 individual units. I bought the standard 9 volt battery type, and not the 10 year lithium battery type for almost twice the price. Also as noted above you may have 1 or 2 carbon monoxide detector, to remain legal these must be replaced with the same type units, I bought these individually. All of these plug into the existing house wiring.
If you are nervous about the wiring, there will be a circuit breaker that you can turn off, a small flat blade screw driver will release the plug from the smoke detector, take it to Lowe's or Home depot, or ACE and get the same model, then reverse the process.
As a side note, one of mine was giving a code that indicated the detector had failed (not a low battery), so that was why I replaced all as I did not want to be bothered over the next year or so replacing the rest of them.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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