Robsteriark
Inside the plipper.
Some (most) plipper casings just prise apart, others need a small crosshead screwdriver. Your owners handbook will show you if you need that information.
As long as you change the battery quickly you will often not need to re-synchronise the plipper to the car. But if the battery is already totally dead or if you remove the old battery and don’t fit the new one within a short time (typically a minute or few) then you will need to re-synchronise the plipper to the car.
Methods for re-sync vary but are usually pretty simple. The method for yours will be in your vehicle handbook, or you can find it online from the Peugeot website. YouTube will almost certainly have relevant content as well.