There's actually a screen you can swipe to to the left of the main screen where you can change the units, but I wanted to make an economic incentive for people to try SI so it'll cost ya! ;-)

Also, I apologize for the buggyness with that swiping... I should have used a UIPageViewController and will probably go fix it at some point.

You can read more about all this in my blog post, including the decision about distance units.