The 'switch button' has a lug on the back flat face - you need to slide a small knife, or similar flat object, between the 'button' and the rim of 'the switch housing' and the 'button' should then pull up and off.
'The switch housing' is simply held in place with a screw which is seen immediately the 'switch button' has been removed.
It always helps to see photos of the parts, which you can at www.buyspares.co.uk. Dyson are not helpful!
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