The carrier has been folded up. See 'Folding the carrier up'. The carrier is not depicted in the figures below to give a clearer view of the parts concerned.
Press the snap fastener (1) in the direction of the arrow and lift the detection chambers a little.
Press the snap fasteners (2) in the direction of the arrow and remove the detection chambers, again in the direction of the arrow.
Position the new detection chambers and install in the direction of the arrow.