Found it. Settings | Dimensions | Units.
However, entering dimensions is tedious because it is necessary to manually clear the existing dimension first, and then enter the new value, not forgetting to enter the unit of measurement as well eg: 2.34m
May I suggest that the original measurement be preselected, so that typing anything immediately replaces the old value; or display nothing and then replace the value when Enter is pressed.
Also, please make the unit of measurement default to what I have selected in Settings.