changing units is not done by a setting, but by editing the layout template files. This is done so that - if you want - you can show metric and imperial both at the same time.
Please read the documentation
on how to do that.
i'm afraid at the moment there is only metrical (km/m) and imperial (mi/ft) available. However it wouldn't be difficult to implement nautical miles and knots - just one more recalculation like for the imperial measures.
I'll put that on my to-do list for the next release!
However it may take like three weeks, because i'm on the road atm.