Unfortunately, the unit on the stock card cannot be modified, so we recommend renaming the stock card in the central warehouse, archiving it and creating a new one with the correct unit. Then don't forget to change the stock card for recipes and also adjust the stock status with new inventory.
You can read more about stock cards at this link: https://help.storyous.com/hc/en-us/articles/360000370889-STOCK-CARDS