To do this task
In the Unit of Measure section, click the drop-down arrow and then do one of the following:
Click an existing unit of measure (Single U/M Per Item mode) or an existing unit of measure set (Multiple U/M Per Item mode).
Click and then complete the wizard to assign a new unit of measure.
Note: If you see the Enable button instead of a drop-down arrow, click Enable and then complete the wizard to turn unit of measure on and assign a unit to this item.