Where can I buy a rubber sheet, a barrel of cooking oil, a swing and 42 ping pong balls?
Very Simple changeout Belle, just shut off the power and match the wires, there will only be two.
I saw a pooch drop a hot potato from its mouth and nuzzle a fire hose.
Hardwired smoke detectors normally have a small plug that goes in the back. If you are really lucky (even buying the same brand), the new unit will take the old plug and base. It is more likely you will have to turn off the power, cut the old plug off or undo the wire nuts and match the new wires to the old. You should find a black, a white, and another color. The black and white are power and neutral and the other color is the connector to the other smoke detectors (if any) in the unit. The interconnect sets off all the detectors if one smells smoke. Hardwired is required by code in many places. I would favor getting a slightly more expensive hardwired with battery backup, which trips the local sound without passing it on if there is a power failure. One reason for losing power would be an electrical fire.
