It's inside the shower unit, at the point of water inlet.
You need to be electrically aware, and technically competent, to safely open the unit and dismantle enough of the shower to get to it, so if you really consider yourself to be clueless then you'd be better off asking the manufacturer to send an engineer.
Another possibility is that you have a scaled-up heater can. This occurs readily in hard water areas, so if the shower isn't new then this one is a strong possibility. You would need a repair engineer for this too.
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