Took delivery of a Headplay HD FPV headset, and not along after, took it apart.

As you do!

 
To help with splitting the two halves, hold the switch panel down with a finger or thumb whilst you separate the halves:

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Inside – you can see the slot where the fresnel lens sits, halfway back towards the rear:

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If you haven’t taken yours apart yet, something to be aware of when re-assembling:

Watch out for the wires, and make sure you don’t pinch the wires with the screw post on the other shell half.

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The manufacturer really should have contained the push button module wiring loom inside a length of flexible “spaghetti tube” to reduce any chance of wiring damage.

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Be careful, when screwing back up again!