How to modify an item's unit of measure

Look up the item via Master Data > Items. Open the item and go to Item Aspects > Units of Measure. To modify the base unit of measure, click on Select base unit of measure and choose a different unit of measure.

Note that it is possible to modify the base unit of measure only when no transactions or order lines exist for the item. The same applies to the secondary base unit of measure. 

On the other hand, input units of measure can be added or modified at any time, regardless of existing transactions and order lines.

For item groups all units of measure can be modified or added at all times, but bear in mind that this may have consquences for the inheritance by items belonging to the item group, including unit configurations.