Open Unit Settings by selecting Unit from the Menu
Select the desired unit type from the scroll list on the top left
Select the desired unit from the scroll list on the middle
Press the unit button you want to change
Press the Add button to add the two units to preferred units, if so desired
To delete a preferred unit, select it and press Delete
Press Apply to accept changes
NOTE: At least one pair of units has to be in preferred units. Therefore you cannot delete it, if only one pair is left.