On the web you can for directions at least, select the "Options" button next to the "Leave Now" time selection, it has a distance units to switch between Automatic / Miles / km. To switch the little scale in the bottom right, just click it.

On the iOS app it's "User"->Settings->Distance Units (I assume Android is similar).

(And if you want to adjust Apple Maps in metric, it is a bit more cryptic, it follows the general system settings app, in General->Language & Region->Measurement System, with a choice of Metric / US / UK).

I find it so frustrating when software and web pages don’t default to using the locale info I’ve set in my system settings. Add an app-specific option if you must, but I’d love if these things just worked by default.

Well, slap my ass 10 times and call me Charles Maurice. When was that introduced?

Charles, it's been there for a decade or more.