You can use the Manage Store module to change the units of measure for any of the items in your inventory. You can add or remove units of measure from the available units used for counts.
Click Store, and then click Manage Store..
Search for and select the item you want to change, and then click Manage Units.
To remove units of measure from an item, in the Included pane, select the units you want to remove, and then click the button with the two right arrows to add them to the Excluded list.
To add units of measure to an item, in the Excluded pane, select the units you want to include, and then click the button with the two left arrows to add to add them to the Included list.
Once you complete unit management for inventory counts, click OK.