How to change metrics to imperial

Change the region where inches are used. Settings > General > Language & Region > Region > [e.g. United States]

Where are you looking to use this?

What country are you in? The Apple Watch gets that setting info from the iPhone settings. If your iPhone region setting is set to US you will get imperial (with US gallons and quarts). If it is set to Canada you will get metric, but unfortunately you can’t enter or see your weight in pounds and feet/inches anymore.

I am terrified if I set it to UK it will come up with stones for weight, whatever they are.