To change the unit of measurement of a stock item, select this stock item in the specification and choose the Change unit of measurement command in the context menu.

This will open a window where you can select a unit of measurement for the selected stock item. You can select stock units of measurement from the same group to which the original unit of measurement belongs. After you select the desired unit of measurement, the window will close. Pressing Cancel will close the window without changing the unit of measurement.