You can change an item type in the item details screen. Items can change to one of 4 types

1. Item = a tradable stock item
2. Comment = a comment line
3. Freight = a freight line
4. Nonexportable = a line that is not to be exported to the retailer

To enter that please select "Menu" >  "Item" > Item Details 

Select the (line) Item code which requires change > Select "Edit" 

Select from the drop down bar "Type" to the specific > Select "Update" > Select "Save"