On my iPhone I can switch Yard - Meter but on my watch I keep seeing Yard. The Knowledge Base says:
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On phone app, click Menu->Settings, toggle "Display Unit" For watch app, open Apple Watch app on phone, click first tab, scroll down to find the TMG app, click in, and enable "use meter" opt ...

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But on that setting screen is no option "use meter", the other lines as showed in the screenshot are all there but no line to switch to meter. (see attached screenshot)