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).