Replacement Smart thermostat to control Hot water

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Ok so I have lost my original smart thermostat that controlled my hot water via the extension kit. I have a replacement smart thermostat which I have added to my devices which works ok for heating. Unfortunately my hot water is still looking for my original thermostat and I dont seem to be able to find how to swap the devices.

So I need to know either

1) how to swap the thermostat associated with a room

2) swap the device selected for hot water control


Many thanks