To edit a food item (in this example Sales Category) select Menu Setup from the top bar of the home screen. Go to Food Items and choose Edit Food Item.
Search for and select the food item from the list on the left. Then select the Sales Category tab.
You will see a standard Two Column Lists. The left side shows all of the Sales Categories in the POS, the right side shows what category the food item is under.
Select the correct category on the left, and click on the incorrect category to deselect it.